This program monitors clock_monotonic and clock_realtime every 10ms
(or a different time lapse passed on the command line).

Whenever the difference between them is more than 0.5ms apart from the
previously-reported difference, it prints the current difference.  All
such differences are relative to the initial offset of the two clocks,
when the program started. The difference is prefixed with the current
date and time, to help logging.

This is something I already had in my toolbox, so I could see the leap
second that occurred on Jun 30th 2012.

Example:
   tornado.root# ./tools/chktime &
   [1] 23761
   ./tools/chktime: looping every 10 millisecs

   tornado.root# ./tools/jmptime .003
   Requesting time-jump: 0.003000 seconds

      13-02-11-12:22:44:       2997 us

   tornado.root# ./tools/jmptime .003
   Requesting time-jump: 0.003000 seconds

      13-02-11-12:22:52:       5994 us

   tornado.root# ./tools/jmptime -.006
   Requesting time-jump: -0.006000 seconds

      13-02-11-12:22:58:        -10 us
Signed-off-by: Alessandro Rubini <rubini@gnudd.com>