Copy head (r256279) to stable/10 as part of the 10.0-RELEASE cycle.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

/* -> /*- for license, minor formatting changes

Added used includes. <sys/_lock.h> is a prerequisite for <sys/_mutex.h>,
and <sys/queue.h> is a prerequisite for both <sys/_lock.h> and
<sys_mutex.h>.

Use NULL instead of __FILE__ in the !LOCK_DEBUG case in the locking code
since the filenames are only used in the LOCK_DEBUG case and are just bloat
in the !LOCK_DEBUG case.

Implement kernel semaphores.

Reviewed by: jhb