chan_iax2: Fix Binding To Multiple Addresses Again

When reworking chan_iax2 for IPv6, the ability to bind to multiple addresses
was removed by mistake.  This patch restores this functionality and adds notes
about IPv6 addresses in the sample config.

(closes issue ASTERISK-22741)
Reported by: Joshua Colp
Tested by: Michael L. Young
Patches:
    asterisk-22741-fix-binding-multiple-addr.diff
                                     uploaded by Michael L. Young (license 5026)

Review: https://reviewboard.asterisk.org/r/2945/
........

Merged revisions 401488 from http://svn.asterisk.org/svn/asterisk/branches/12


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@401489 65c4cc65-6c06-0410-ace0-fbb531ad65f3