Fix error in the "IPorHost" section of a SIP dialstring.

Fix error in the "IPorHost" section of a SIP dialstring.

This is based on the review request posted by Walter Doekes
(referenced lower in the commit message)

The main fix here is to treat the IPorHost portion of the dial
string as a temporary outbound proxy. This ensures requests
get sent to the proper location.

Due to the age of the request, some parts were no longer relevant.
For instance, the request moved outbound proxy parsing code into
a single method. This is done in a previous commit, so it was not
necessary to do again.

Also, the review request fixed some errors with regards to request
routing for CANCEL and ACK requests. This has also been fixed in
more recent commits.

(closes issue ASTERISK-19677)
reported by Walter Doekes

Review https://reviewboard.asterisk.org/r/1859
........

Merged revisions 370769 from http://svn.asterisk.org/svn/asterisk/branches/1.8

git-svn-id: https://origsvn.digium.com/svn/asterisk/certified/branches/1.8.11@370843 65c4cc65-6c06-0410-ace0-fbb531ad65f3