hwclock: use parse_date function

hwclock: use parse_date function

* hwclock.c: replace interpret_date_string() with parse_date().
   Eliminates shell injection vulnerability from popen() date(1).
   Removes system dependency on date(1).
   Reduction in hwclock execution time.
   Significant cleanup of hwclock.c code.

* Bug fix for "hwclock --predict --date 'bad_date'" printing:
    hwclock: No usable set-to time.  Cannot set clock.

  The message now matches date(1) for an invalid --date argument.

Signed-off-by: J William Piggott <elseifthen@gmx.com>