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.
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Merged revisions 381668 from http://svn.asterisk.org/svn/asterisk/branches/1.8
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Merged revisions 381669 from http://svn.asterisk.org/svn/asterisk/branches/11


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@381670 65c4cc65-6c06-0410-ace0-fbb531ad65f3