Remove "registertrying" and add "rtp_engine" from/to sip.conf.sample

The "registertrying" option was removed in r343220. The "rtp_engine"
option was added in r186078 but erroneously named "engine" in the sample.
Note that there is no global sip setting for a different engine.


git-svn-id: http://svn.asterisk.org/svn/asterisk/branches/1.8@381668 f38db490-d61c-443f-a65b-d21fe96a405b