Fix our compliance with RFC 3261 section 18.2.2.

Fix our compliance with RFC 3261 section 18.2.2.

This change optimizes the free_via() function and removes some redundant null
checking. It also fixes compliance with RFC 3261 section 18.2.2 by always using
the port specified in the Via header for routing responses (even when maddr is
not set). Also the htons() function is now used when setting the port.
Additional documentation comments have been added in various places to make the
logic in the code clearer.

(closes issue #18951)
Reported by: jmls
Patches:
      issue18951_set_proper_port_from_via.patch uploaded by wdoekes (license 717) (modified)

git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/1.4@315891 65c4cc65-6c06-0410-ace0-fbb531ad65f3