Retires certain usernames.

This patch retires the following usernames:

*  dmr - Dennis MacAlistair Ritchie (September 9, 1941 - October 12, 2011)
*  mrc - Mark Reed Crispin (July 19, 1956 - December 28, 2012)
*  jmc - John McCarthy (September 4, 1927 – October 24, 2011)