[PATCH] Detect GNU/kFreeBSD in user-visible kernel headers

Hi!

Out of the kernel headers that are installed in /usr/include/ hierracy, there
are some which include support multiple operating systems (usually FreeBSD and
other *BSD flavours).

This patch adds support to detect GNU/kFreeBSD as well.  In all cases, we
match the same declarations as FreeBSD does (which is to be expected in kernel
headers, since both systems share the same kernel).

Does it look fine?