UPGRADE: Note IAX2 compatibility issue between 1.4 and 1.8+ systems.

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Merged revisions 411457 from http://svn.asterisk.org/svn/asterisk/branches/1.8
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Merged revisions 411458 from http://svn.asterisk.org/svn/asterisk/branches/11
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Merged revisions 411459 from http://svn.asterisk.org/svn/asterisk/branches/12


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@411460 65c4cc65-6c06-0410-ace0-fbb531ad65f3