TIMER: Add correct sim_activate_time functionality for all timer cases.

Also:
- allow a timer to dynamically stop itself (by calling sim_rtcn_calb
   with tps=0), and then to start the internal timer to provide a calibration
   baseline if necessary.
- Fix coschedule interval computation when the queue is empty.
- Properly select the correct timer for coscheduling without a specific tmr
- Properly adjust the coschedule queue's next time value when entries are
   canceled.
- Cleaned up throttling to behave well with all potential throttling rates and
   to make visible what is happening when boundaries are reached.

scp.h

@@ -169,6 +169,7 @@ t_stat fprint_val (FILE *stream, t_value val, uint32 rdx, uint32 wid, uint32 fmt
 t_stat sprint_val (char *buf, t_value val, uint32 rdx, uint32 wid, uint32 fmt);
 t_stat sim_print_val (t_value val, uint32 radix, uint32 width, uint32 format);
 const char *sim_fmt_secs (double seconds);
+const char *sim_fmt_numeric (double number);
 const char *sprint_capac (DEVICE *dptr, UNIT *uptr);
 char *read_line (char *cptr, int32 size, FILE *stream);
 void fprint_reg_help (FILE *st, DEVICE *dptr);