Add futex(2) syscall based on a sane subset of its Linux equivalent.

Add futex(2) syscall based on a sane subset of its Linux equivalent.

The syscall is marked NOLOCK and only FUTEX_WAIT grabs the KERNEL_LOCK()
because of PCATCH and the signal nightmare.

Serialization of threads is currently done with a global & exclusive
rwlock.

Note that the current implementation still use copyin(9) which is not
guaranteed to be atomic.  Committing now such that remaining issues can
be addressed in-tree.

With inputs from guenther@, kettenis@ and visa@.

ok deraadt@, visa@