Add syscall primitives for generic userland accessible sleep/wakeup

Add syscall primitives for generic userland accessible sleep/wakeup
functions.  These functions are capable of sleeping and waking up based on
a generic user VM address.  Programs capable of sharing memory are also
capable of interaction through these functions.

Also regenerate our system calls.

umtx_sleep(ptr, matchvalue, timeout)

    If *(int *)ptr (userland pointer) does not match the matchvalue,
    sleep for timeout microseconds.  Access to the contents of *ptr plus
    entering the sleep is interlocked against calls to umtx_wakeup().
    Various error codes are turned depending on what causes the function
    to return.  Note that the timeout may not exceed 1 second.

utmx_wakeup(ptr, count)

    Wakeup at least count processes waiting on the specified userland
    address.  A count of 0 wakes all waiting processes up.  This function
    interlocks against umtx_sleep().

The typical race case showing resolution between two userland processes is
shown below.  A process releasing a contested mutex may adjust the contents
of the pointer after the kernel has tested *ptr in umtx_sleep(), but this does
not matter because the first process will see that the mutex is set to a
contested state and will call wakeup after changing the contents of the
pointer.  Thus, the kernel itself does not have to execute any
compare-and-exchange operations in order to support userland mutexes.

    PROCESS 1			PROCESS 2		******** RACE#1 ******

    cmp_exg(ptr, FREE, HELD)
	.			cmp_exg(ptr, HELD, CONTESTED)
	.			umtx_sleep(ptr, CONTESTED, 0)
	.			[kernel tests *ptr]     <<<< COMPARE vs
    cmp_exg(CONTESTED, FREE)		.		<<<< CHANGE
	.			tsleep(....)
    umtx_wakeup(ptr, 1)			.
	.				.
	.				.

    PROCESS 1			PROCESS 2		******** RACE#2 ******

    cmp_exg(ptr, FREE, HELD)
				cmp_exg(ptr, HELD, CONTESTED)
				umtx_sleep(ptr, CONTESTED, 0)
    cmp_exg(CONTESTED, FREE)				<<<< CHANGE vs
    umtx_wakeup(ptr, 1)
				[kernel tests *ptr]	<<<< COMPARE
				[MISMATCH, DO NOT TSLEEP]

These functions are very loosely based on Jeff Roberson's umtx work in
FreeBSD.  These functions are greatly simplified relative to that work in
order to provide a more generic mechanism.

This is precursor work for a port of David Xu's 1:1 userland threading
library.