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. This 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.

This commit also adds/corrects the aliases from the last one.