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.

lib/libsys/Makefile

@@ -59,6 +59,7 @@ SRCS+=  \
 	sys_schedule.c \
 	sys_setalarm.c \
 	sys_setgrant.c \
+	sys_settime.c \
 	sys_sigreturn.c \
 	sys_sigsend.c \
 	sys_sprof.c \

lib/libsys/sys_settime.c

@@ -0,0 +1,20 @@
+#include "syslib.h"
+#include <time.h>
+
+int sys_settime(now, clk_id, sec, nsec)
+int now;
+clockid_t clk_id;
+time_t sec;
+long nsec;
+{
+  message m;
+  int r;
+
+  m.T_SETTIME_NOW = now;
+  m.T_CLOCK_ID = clk_id;
+  m.T_TIME_SEC = sec;
+  m.T_TIME_NSEC = nsec;
+
+  r = _kernel_call(SYS_SETTIME, &m);
+  return(r);
+}