Constants for the linux nfqueue netlink protocol (userspace firewall).

Continuation of #1562 and #1571.

Ref: https://android.googlesource.com/platform/bionic/+/refs/tags/ndk-r20/libc/include/sys/ucontext.h

Ref: https://android.googlesource.com/platform/bionic/+/refs/tags/ndk-r20/libc/include/sys/ucontext.h

Ref: https://android.googlesource.com/platform/bionic/+/refs/tags/ndk-r20/libc/include/sys/ucontext.h

This triggers a warning on a recent nightly, which in turn breaks CI due to
`#![deny(warnings)]` in libc-test/build.rs

Signed-off-by: Arthur Gautier <baloo@gandi.net>

First batch for #1562

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

Closes #659