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.

Moved inotify_rm_watch out of the generic notbsd target and into specific linux and android targets because the second argument has a different type in android and linux.

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 is an error from PR #1098.  The wrong definitions coincidentally
work on Linux/glibc, but fail on Linux/musl.

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

On FreeBSD, uname is an inline function.  The uname that is present in
libc.so is for FreeBSD 1.0 compatibility.  It expects a buffer of a
different size.

Fixes #1190
Reported-by: Alex Zepeda

Fixes #1017

I moved it up to src/unix/mod.rs, as it's specified in POSIX.1-2008 and
appears to be implemented on every Unix-like system.

The symbol names on macOS appear similar to those for opendir; I found
them via the commands below. I tested the x86_64 version;
fdopendir$INODE64 worked as expected.

$ nm -arch x86_64 /usr/lib/system/libsystem_c.dylib | grep fdopendir
000000000007ea6d T _fdopendir
000000000002ba97 T _fdopendir$INODE64
$ nm -arch i386 /usr/lib/system/libsystem_c.dylib | grep fdopendir
00082d1e T _fdopendir
0002b528 T _fdopendir$INODE64$UNIX2003
00082d1e T _fdopendir$UNIX2003