Recognize Debian GNU/kFreeBSD as FreeBSD-ish.

Recognize Debian GNU/kFreeBSD as FreeBSD-ish.

Some of the FreeBSD-specific code in libwebsockets is related to
the FreeBSD kernel, not the general build environment.  Thus, it is
important to make this distinction, especially when building on
platforms that have a FreeBSD kernel and a non-FreeBSD userland build
environment, such as Debian GNU/kFreeBSD.

When checking for FreeBSD kernel features, also check for the newly
introduced __FreeBSD_kernel__ preprocessor constant; it is present in
the GNU/kFreeBSD kernel and also in FreeBSD itself since the 9.1 release
about three years ago.