Fix Displaying Symmetric RTP Global Setting

* Use comedia_string() to display correctly the symmetric rtp setting when
  running "sip show settings"
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Merged revisions 386486 from http://svn.asterisk.org/svn/asterisk/branches/11


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@386487 65c4cc65-6c06-0410-ace0-fbb531ad65f3