Currently, libc supports and detects freebsd11 and freebsd13
Unknown versions, like freebsd13, is treated as freebsd11.
This patch solve the issues, detecting freebsd13 and treating it like
freebsd12.
Inverting the logic not(freebsd12) -> freebsd11 where possible

Signed-off-by: Diana Popa <dpopa@amazon.com>

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

Per discussion in #1410, this is necessary to avoid struct size mismatches between Rust and C on
systems with glibc < 2.28.

Add cfg_attr

Closes #659

PR #849 just moved all of the Linux structures into `src/fuchsia`.
While this is good for the most part, we really don't want the `SYS_*`
constants exported on Fushia, as these are not Fuchsia's syscalls.

This also removes Linux-specific `GRND_*` constants.

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

During the refactor in 4bd419eb, PTRACE_DETACH was accidentally removed on
these targets. This re-adds it.