Store SIP User-Agent information in contacts.

When an endpoint sends a REGISTER request to Asterisk, we now will
associate the User-Agent header with all contacts that were bound in
that REGISTER request.
........

Merged revisions 408270 from http://svn.asterisk.org/svn/asterisk/branches/12


git-svn-id: http://svn.asterisk.org/svn/asterisk/trunk@408272 f38db490-d61c-443f-a65b-d21fe96a405b