Incorporate feedback from bde and jilles regarding r265472 to dd(1).

* Don't use sysexits.h.  Just exit 1 on error and 0 otherwise.
* Don't sacrifice precision by converting the output of clock_gettime() to a
  double and then comparing the results.  Instead, subtract the values of
  the two clock_gettime() calls, then convert to double.
* Don't use CLOCK_MONOTONIC_PRECISE.  It's an unportable synonym for
  CLOCK_MONOTONIC.
* Use more appropriate names for some local variables.
* In the summary message, round elapsed time to the nearest microsecond.

Reported by:	bde, jilles
MFC after:	3 days
X-MFC-With:	265472