Add real sched.h

It is required by IEEE Std 1003.1-2008 AKA POSIX.

Put some Linux compatibility stuff under BSD_VISIBLE namespace, in
particular, sys/cpuset.h definitions. Also, if user really want
Linux compatibility, she can request cpu_set_t typedef with
_WITH_CPU_SET_T define.

Reviewed by: jhb
Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D32901