Don't reference sem(4) from the POSIX semaphore pages.  POSIX semaphores
were reimplemented using umtx in FreeBSD 9 and no longer use sem(4).
HPET: avoid handling the multiple file-descriptor case.

It had two bugs: one where mmap was still allowed and another where
D_TRACKCLOSE doesn't handle all cases.

Thanks to jhb and kib for pointing them out.
MFC after:	1 week
Add COMPAT_FREEBSD9 and COMPAT_FREEBSD10 options to wrap code that
provides compatability for FreeBSD 9.x and 10.x binaries.  Enable
these options in kernel configs that enable other COMPAT_FREEBSD<n>
options.