libc: add clock_settime() system call.

This also adds the sys_settime() kernel call which allows for the adjusting
of the clock named realtime in the kernel. The existing sys_stime()
function is still needed for a separate job (setting the boottime). The
boottime is set in the readclock driver. The sys_settime() interface is
meant to be flexible and will support both clock_settime() and adjtime()
when adjtime() is implemented later.

settimeofday() was adjusted to use the clock_settime() interface.

One side note discovered during testing: uptime(1) (part of the last(1)),
uses wtmp to determine boottime (not Minix's times(2)). This leads `uptime`
to report odd results when you set the time to a time prior to boottime.
This isn't a new bug introduced by my changes. It's been there for a while.

kernel/system.c

@@ -235,6 +235,7 @@ void system_init(void)
   map(SYS_TIMES, do_times);		/* get uptime and process times */
   map(SYS_SETALARM, do_setalarm);	/* schedule a synchronous alarm */
   map(SYS_STIME, do_stime);		/* set the boottime */
+  map(SYS_SETTIME, do_settime);		/* set the system time (realtime) */
   map(SYS_VTIMER, do_vtimer);		/* set or retrieve a virtual timer */
 
   /* System control. */