Adding OpenBSD as per request, verified here: https://github.com/openbsd/src/blob/master/sys/sys/stat.h#L188

Exposes value for most unix like platforms

These constants have already been deprecated for a few releases with
a deprecation notice, so they can finally be removed.

Closes rust-lang/libc#665

sigevent structs on most platforms have padding or unused fields. Rather
than display those in the Debug impl by deriving it, manually implement
all extra_traits instead ignoring those fields.

* Change the type of FIONCLEX on apple platforms from c_uint to c_ulong
* Add FIONCLEX, FIONREAD, FIOASYNC, FIOSETOWN and FIOGETOWN
  for DragonFly and OpenBSD

This commits adds a second FreeBSD 12 build job,
and splits the implementation of the FreeBSD module
into two modules, one for FreeBSD 11, and one for FreeBSD 12.

The FreeBSD 11 module is compiled always by default, and is
mostly forward compatible with FreeBSD 12 systems.

The FreeBSD 12 module is only built for now in libc's CI,
and uses FreeBSD 12 data types and APIs, linking to symbols
that are only available in FreeBSD 12.

Basically, when LIBC_CI env variable is defined, and the host
system is a FreeBSD 12 system, then the FreeBSD 12 module is
automatically built and tested. Conditional compilation is done
using a `cfg(freebsd12)` flag.

This commit also re-enables many tests, and documents why
some remain disabled.

The second argument of `gettimeofday` was a `*mut c_void` on all targets,
but that type is incorrect in the following targets, where it should be
a `*mut timezone` instead:

On these other targets it appears that the signature of gettimeofday was incorrect (it takes a time-zone pointer instead of a void pointer):

linux+gnu: http://man7.org/linux/man-pages/man2/gettimeofday.2.html
freebsd: https://www.freebsd.org/cgi/man.cgi?query=gettimeofday&apropos=0&sektion=2&manpath=FreeBSD+11.2-stable&arch=default&format=html
openbsd: https://man.openbsd.org/gettimeofday.2
android: https://github.com/ricardoquesada/android-ndk/blob/master/usr/include/sys/time.h
dragonfly: https://www.dragonflybsd.org/cgi/web-man?command=gettimeofday&section=2

This commit corrects the type on these targets, which is a breaking change. Due
to how this API is commonly used (e.g. passing `ptr::null_mut` to the second
argument), breakage should be minimal. Users wanting to support both versions
can just write `ptr as *mut _` instead.

Closes #1338.

See https://github.com/rust-lang/rust/pull/60775 .

while here, generate the right file in test_openbsd()

This cleans up the build.rs of `libc-test` for apple targets.

I wanted to update the docker containers of some targets so that we can start
testing newer currently-skipped APIs properly, but it is impossible to figure
out which headers and APIs are skipped for each target.

This PR separates the testing of apple targets into its own self-contained
function. This allows seeing exactly which headers are included, and which items
are skipped. A lot of work will be required to separate the testing of all major
platforms and make the script reasonable.

During the clean up, I discovered that, at least for apple targets, deprecated
but not removed APIs are not tested. I re-enabled testing for those, and fixed
`daemon`, which was not properly linking its symbol. I also added the
`#[deprecated]` attribute to the `#[deprecated]` APIs of the apple targets. The
attribute is available since Rust 1.9.0 and the min. Rust version we support is
Rust 1.13.0.

Many other APIs are also currently not tested "because they are weird" which I
interpret as "the test failed for an unknown reason", as a consequence:

* the signatures of execv, execve, and execvp are incorrect (see
  https://github.com/rust-lang/libc/issues/1272)

* the `sig_t` type is called `sighandler_t` in libc for some reason:
  https://github.com/rust-lang/libc/issues/1273

This probably explains why some other things, like the
`sa_handler`/`sa_sigaction` fields of `sigaction` were skipped. The field is
actually a union, which can be either a `sig_t` for the `sa_handler` field, or
some other type for the `sa_sigaction` field, but because the distinction was
not made, the field was not checked.

The latest ctest version can check volatile pointers, so a couple of skipped
tests are now tested using this feature.

I guess the previous lint failure was about avoiding #[cfg], and doing
this instead.

Will likely alter this after seeing CI results across platforms.

This PR fixes the build on all platforms and all Rust version down to the
minimum Rust version supported by libc: Rust 1.13.0.

The `build.rs` is extended with logic to detect the newer Rust features used by
`libc` since Rust 1.13.0:

* Rust 1.19.0: `untagged_unions`. APIs using untagged unions are gated on
  `cfg(libc_unions)` and not available on older Rust versions.

* Rust 1.25.0: `repr(align)`. Because `repr(align)` cannot be parsed by older
  Rust versions, all uses of `repr(align)` are split into `align.rs` and
  `no_align.rs` modules, which are gated on the `cfg(libc_align)` at the top
  level. These modules sometimes contain macros that are expanded at the top
  level to avoid privacy issues (`pub(crate)` is not available in older Rust
  versions). Closes #1242 .

* Rust : `const` `mem::size_of`. These uses are worked around with hardcoded
  constants on older Rust versions.

Also, `repr(packed)` structs cannot automatically `derive()` some traits like
`Debug`. These have been moved into `s_no_extra_traits!` and the lint of missing
`Debug` implementations on public items is silenced for these. We can manually
implement the `extra_traits` for these in a follow up PR. This is tracked
in #1243. Also, `extra_traits` does not enable `align` manually anymore.

Since `f64::to_bits` is not available in older Rust versions, its usage
has been replaced with a `transmute` to an `u64` which is what that method
does under the hood.

Closes #1232 .

This was not compile-tested on all platforms, but instead all `pub enum`
types had a `Debug` impl derived for them.

PR #1098 added the CMSG_DATA(3) family of functions into libc.  Because
they're defined as macros in C, they had to be rewritten as Rust
functions for libc.  Also, they can't be tested in CI for the same
reason.  But that PR erroneously used the same definitions in BSD as in
Linux.

This commit corrects the definitions for OSX, FreeBSD, DragonflyBSD,
OpenBSD, and NetBSD.  I renamed a few variables and collapsed a few
macros in order to combine the definitions where possible.

Fixes #1210

The aio functions require librt on NetBSD.