channels/chan_sip: Support mutltiple Supported and Required headers

channels/chan_sip: Support mutltiple Supported and Required headers

A SIP request may contain multiple Supported: and Required: headers. Currently,
chan_sip only parses the first Supported/Required header it finds. This patch
adds support for multiple Supported/Required headers for INVITE requests.

Review: https://reviewboard.asterisk.org/r/2478

ASTERISK-21721 #close
Reported by: Olle Johansson
patches:
  rb2478.patch uploaded by oej (License 5267)
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Merged revisions 426594 from http://svn.asterisk.org/svn/asterisk/branches/1.8
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Merged revisions 426595 from http://svn.asterisk.org/svn/asterisk/branches/11
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Merged revisions 426596 from http://svn.asterisk.org/svn/asterisk/branches/12

git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/13@426597 65c4cc65-6c06-0410-ace0-fbb531ad65f3