channels/chan_sip: Respect outboundproxy setting when sending qualify requests

The outboundproxy setting is currently ignored when sending OPTIONS requests
as a result of the qualify setting. This means that if an Asterisk server is
unable to send the packet directly to a peer, it is unable to qualify any
non-inbound registered peer (e.g. a peer SIP Trunk).

This patch grabs the outboundproxy information for a peer when a qualify
attempt is being constructed and, if it finds the information, uses it
when sending the OPTIONS request.

Review: https://reviewboard.asterisk.org/r/3948

ASTERISK-24063 #close
Reported by: Damian Ivereigh
patches:
  outboundproxy-dai.patch uploaded by Damian Ivereigh (License 6632)
........

Merged revisions 425818 from http://svn.asterisk.org/svn/asterisk/branches/1.8


git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/11@425819 65c4cc65-6c06-0410-ace0-fbb531ad65f3