Support was previously added to gnu x86_64 and all musl targets but
others were not included because of a roundtrip issue [1].

This commit adds support for the ip_mreqn struct on all Linux GNU
targets which did not have the roundtrip issue.

[1]: https://github.com/rust-lang/libc/issues/1558



Signed-off-by: Bartel Sielski <bartel.sielski@gmail.com>

Reorganized some constants as well so they're grouped a bit better.

These are used by the flock wrapper in rustc_data_structures. The
constants were already present in x86_64-linux-gnu and BSD (since
4928bd98).

`pthread_mutex_t` varies across architectures, in several ways:
- endianness alters the ordering of bytes, since the contents of the
  struct are larger than 8-bit.
- its length varies.
- the location of the mutex kind (`PTHREAD_MUTEX_RECURSIVE`,
  `PTHREAD_MUTEX_ERRORCHECK` or `PTHREAD_MUTEX_ADAPTIVE_NP`) varies
  between 32-bit and 64-bit: On 32-bit architectures, it is preceded by
  three int/unsigned int, while on 64-bit architectures, it is preceded
  by four of them.

These initializers are only available from <pthread.h> when _GNU_SOURCE
is defined.

Relax the cfg_if check in ci/style.rs to allow #[cfg(target_endian)]
tests.

Following types and function have been added:
* pthread_cond_init
* pthread_condattr_t
* pthread_condattr_init
* pthread_condattr_destroy
* pthread_condattr_setclock [0]
* pthread_condattr_getclock [0][1]

[0] Not available on OSX.
[1] May be unavailable on NetBSD, it is a very recent addition.
    Left out from openbsdlike hierarchy for now.

Sadly, the sysinfo struct varies slightly between Musl and Glibc / Bionic.

This means that users need to be careful when using the uptime, and should
always cast it to a signed value. Why uptime can be signed is beyond me...

This commit changes most uses of cfg_if as follows:
- fallthrough `else` usage is avoided for architecture or OS specific
  items
- a comment is added in the final `else` clause to signal intent someone
  modifying

It is safer to omit items than include ones for the wrong platform or
architecture.

It's tough to have PRs bounce or to have a back and forth with contributors
about minor style quibbles. Sometimes it ends up just being easier to fix style
after the fact, but let's add some automation to help this!

This commit adds a script to run on CI and locally to verify the style of this
repository. There's a few stylistic guidelines to ensure that definitions are
understandable across the jungle of modules. This consistency should help assist
readability for any future readers!

This is a blanket import of lots of constants and function from nix-rust

Fixes a few constants along the way

They're all added under the `foo64` names with the `foo64` types added as well,
so they still need to be explicitly chosen.