Fix logic regarding when to perform an SRV lookup for outgoing REGISTER requests

Fix logic regarding when to perform an SRV lookup for outgoing REGISTER requests

With this fix, we only will perform an SRV lookup at the following times:

* The first time we register with a remote registrar
* If we send a REGISTER but do not receive a response
* If the sendto() function returns an error

While I wrote the patch that fixes this issue, a huge amount of credit is due
to Brett Bryant, who wrote the initial patch on which I based this one.

(closes issue #12312)
Reported by: jrast
Patches:
      12312.patch uploaded by putnopvut (license 60)
Tested by: blitzrage

Review: http://reviewboard.digium.com/r/132/

git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/1.4@173770 65c4cc65-6c06-0410-ace0-fbb531ad65f3