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.h

@@ -164,6 +164,7 @@ int do_setalarm(struct proc * caller, message *m_ptr);
 #endif
 
 int do_stime(struct proc * caller, message *m_ptr);
+int do_settime(struct proc * caller, message *m_ptr);
 
 int do_vtimer(struct proc * caller, message *m_ptr);
 #if ! USE_VTIMER