Allow specifying a port number in the user portion of a register => line in sip.conf

Allow specifying a port number in the user portion of a register => line in sip.conf

With this commit, a register => line in sip.conf may contain a port number in the
"user" section of the line. Please see CHANGES and sip.conf.sample for more
details regarding this.

(closes issue #14198)
Reported by: Nick_Lewis
Patches:
      chan_sip.c-domainport2.patch uploaded by Nick (license 657)
Tested by: Nick_Lewis

git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@168575 65c4cc65-6c06-0410-ace0-fbb531ad65f3