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

The aio functions require librt on NetBSD.

OpenBSD doesn't implement fexecve. The only reference of it that I can
find in the OpenBSD source is in the man pages of signal(3) and
sigaction(2) (where it's mentioned that it is not implemented).

OpenBSD official source code link:
https://cvsweb.openbsd.org/src/lib/libc/sys/sigaction.2?rev=1.75&content-type=text/x-cvsweb-markup

OpenBSD Github mirror:
https://github.com/openbsd/src/blob/master/lib/libc/sys/sigaction.2#L619

On NetBSD's unistd.h I see that it is under an ifdef. Calling it returns
78 / ENOSYS / Function not implemented.

NetBSD office source code link:
http://cvsweb.netbsd.org/bsdweb.cgi/src/include/unistd.h?rev=1.151&content-type=text/x-cvsweb-markup&only_with_tag=MAIN

NetBSD Github mirror:
https://github.com/NetBSD/src/blob/trunk/include/unistd.h#L319

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 are defined in the fcntl.h glibc header on s390x systems
on Linux but missing in the libc crate, so add them as they are
required for the file locking API in rustc.

These are defined in the fcntl.h glibc header on MIPS systems
on Linux but missing in the libc crate, so add them as they are
required for the file locking API in rustc.

separate for gnu and msvc


scope resolve c_int

these types are not allowed, and more scope resolution


use size_t