Correct two flaws in sip.conf.sample related to AST-2011-013.
* The sample file listed *two* values for the 'nat' option as being the default.
Only 'force_rport' is the default.
* The warning about having differing 'nat' settings confusingly referred to both
peers and users.
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Merged revisions 348515 from http://svn.asterisk.org/svn/asterisk/branches/1.6.2
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Merged revisions 348516 from http://svn.asterisk.org/svn/asterisk/branches/1.8
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Merged revisions 348517 from http://svn.asterisk.org/svn/asterisk/branches/10
git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@348518
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