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.

dragonflybsd still uses c_char [2]

[1] https://svnweb.freebsd.org/base/releng/11.2/sys/sys/signal.h?revision=334459&view=markup#l438
[2] https://github.com/DragonFlyBSD/DragonFlyBSD/blob/master/sys/sys/signal.h#L339

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

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

These errno's are defined in <sys/errno.h> on DragonFlyBSD.

Signed-off-by: Levente Kurusa <lkurusa@acm.org>

Also, alphabetize this section, since I'm merging two sections anyway.

Fixes #1113

DragonflyBSD does define 3 of these 14 functions.  But I elected not to
add it to the PR because I can't find any evidence that these syscalls
are used on Dragonfly, even in the base system.  And by themselves,
those three are insufficient for the xattr crate.

* Added ptrace and constants for all freebsdlike and netbsdlike OSs

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

Signed-off-by: Arvid E. Picciani <aep@exys.org>

Sorry, accidentally forgot this when submitting #934..

This follows the same method as other platforms like OSX and NetBSD.

This will fix rustup and building from git (once libc is updated for bootstrap)
on FreeBSD12 post-ino64 in https://github.com/freebsd/freebsd/commit/f713b08c027de35b0b8ed9990eabc32e100bee7a.
It also avoids having to hotpatch the stage0 compiler, and HOME/.cargo
libc files on FreeBSD12 to build rust.

The only real pitfall is that this will prevent interaction with inodes that
have an ino_t above the 32-bit limit due to truncation.  On the other hand
Rust won't work at all on 12 without doing this currently.  In general
it should not be a problem for users and if they need 64-bit ino_t they
can use a patched libc, rather than the current state of affairs in
requiring a patched libc to use Rust on 12.

A better, or complementary, approach would be something like proposed in
https://github.com/rust-lang/rfcs/pull/2048 to allow targetting a specific
version of FreeBSD. This would allow Rust to default to this compatibility
mode by targetting FreeBSD10 and still allow targetting FreeBSD12 for 64-bit
ino_t.

The symbol versions used were taken from the old version in
https://github.com/freebsd/freebsd/commit/f713b08c027de35b0b8ed9990eabc32e100bee7a#diff-61a32fcfb7ecd4517665fed591813c57
and
https://github.com/freebsd/freebsd/commit/f713b08c027de35b0b8ed9990eabc32e100bee7a#diff-7f67ccf8b5f44ff2f54eaab0207abb8d.

The scope of functions versioned here differs from other platforms as
not all structs were modified that were on others, such as DIR for
`opendir`, `telldir`, etc.  Only functions using dirent, stat, glob_t,
and dev_t need the changes.

Fixes https://github.com/rust-lang/rust/issues/42681

struct kevent was modified in FreeBSD12.  The @FBSD_1.0 symbol supports the old
structure ABI still.

This allows the `mio` crate tests to now pass on FreeBSD12.