Fix #defines for Debian GNU/kFreeBSD

Because debian does not run with the freebsd libc, but rather uses the
GNU one, it chose to not define __FreeBSD__, but rather __FreeBSD_kernel__.
Use the alternative when the functionality tested is for kernel
features, and keep the __FreeBSD__ one when using freebsd libc
headers.

If this patch is applied, debian could drop all the current patches when
importing 6.0 :)