libs/bsd: Implement getentropy(2).

With generic_syscall to avoid using a FD.

Change-Id: Id1702118bb673f06a861e4f0ad3401ce3d45cbd0
Reviewed-on: https://review.haiku-os.org/c/haiku/+/31
Reviewed-by: waddlesplash <waddlesplash@gmail.com>
Tested-by: Commit checker robot <no-reply+buildbot@haiku-os.org>
Reviewed-by: Adrien Destugues <pulkomandy@pulkomandy.tk>