libbsd: add pthread_sigqueue()

* like sigqueue() but for threads. was added recently to FreeBSD, long before to glibc.
* also include features.h in gnu/pthread.h

Change-Id: I73bca666e2c67d7ff05f341bf85d71df9ccccbe5
Reviewed-on: https://review.haiku-os.org/c/haiku/+/7659
Reviewed-by: waddlesplash <waddlesplash@gmail.com>
Tested-by: Commit checker robot <no-reply+buildbot@haiku-os.org>