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

Available since FreeBSD 9.0, this API allows using local descriptors
instead of global PIDs. It works in Capsicum sandboxes.

rustc, an important libc consumer, has a policy that they can't use any
crates with deprecated symbols.  Replace libc's two deprecated symbols
with hidden symbols instead.

Note that OpenBSD and OSX do not support POSIX message queues.

This was not caught by libc-test, as checks for the execv functions
are disabled.

This reverts commit c124acba.

Add network interface flag constants for all platforms.

Rebase of #610 and also move emscripten up much higher in the hierarchy to
ensure that it doesn't have too much of a ripple effect on other platforms.

This involved moving down a good number of definitions, but hopefully was done
with care to not break anything!

This doesn't appear on any other platforms

These constants aren't available from the system headers so don't expose
them here.