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/Makefile.inc

@@ -13,6 +13,7 @@ SRCS+= 	\
 	do_times.c \
 	do_setalarm.c \
 	do_stime.c \
+	do_settime.c \
 	do_vtimer.c \
 	do_irqctl.c \
 	do_copy.c \

kernel/system/do_settime.c

@@ -0,0 +1,43 @@
+/* The kernel call implemented in this file:
+ *   m_type:	SYS_SETTIME
+ *
+ * The parameters for this kernel call are:
+ *    m4_l2:	T_SETTIME_NOW
+ *    m4_l3:	T_CLOCK_ID
+ *    m4_l4:	T_TIME_SEC
+ *    m4_l5:	T_TIME_NSEC
+ */
+
+#include "kernel/system.h"
+#include <minix/endpoint.h>
+#include <time.h>
+
+/*===========================================================================*
+ *				do_settime				     *
+ *===========================================================================*/
+int do_settime(struct proc * caller, message * m_ptr)
+{
+  clock_t newclock;
+  time_t timediff;
+
+  if (m_ptr->T_CLOCK_ID != CLOCK_REALTIME) /* only realtime can change */
+	return EINVAL;
+
+  /* prevent a negative value for realtime */
+  if (m_ptr->T_TIME_SEC <= boottime) {
+  	/* boottime was likely wrong, try to correct it. */
+	boottime = m_ptr->T_TIME_SEC;
+	set_realtime(1);
+	return(OK);
+  }
+
+  /* calculate the new value of realtime in ticks */
+  timediff = m_ptr->T_TIME_SEC - boottime;
+  newclock = (timediff*system_hz) + (m_ptr->T_TIME_NSEC/(1000000000/system_hz));
+
+  if (m_ptr->T_SETTIME_NOW) {
+	set_realtime(newclock);
+  } /* else used adjtime() method (to be implemented) */
+
+  return(OK);
+}