-Get process group ID (pgid) instead of process ID. It didn't work when
 it was called from cron.
-pgrep should accept full path. Without "-f" pgrep accepts "ntpd", but
 not "/usr/sbin/ntpd"

Because of the two reasons above the system_clock_monitor script was not
killing ntpd correctly.

In practice, the ntp will be killed after 10s, instead of default
timeout from ntpd 50s.

Signed-off-by: Adam Wujek <dev_public@wujek.eu>