check_ntp_time: adding offset option

Not sure if this is of use or not - we have a strange requirement to run
certain servers 5 minutes fast. I've added a switch to the check_ntp_time
to allow for this offset.

Thanks to Patrick McAndrew.
Adding Patrick McAndrew to THANKS.in
check_ntp_time.c - Cleaned up usage and added a note

Closes #907
Closes #1184