Prevent potential infinite outbound authentication loops in registration.

Prevent potential infinite outbound authentication loops in registration.

Prior to this patch, Asterisk would always respond to 401 responses to
registration attempts by trying to provide a registration with authentication
credentials. Even if subsequent attempts were rejected with 401 responses,
Asterisk would continue this behavior. If authentication credentials were
incorrect, this could continue forever.

With this patch, we keep track of whether we have attempted authentication
on an outbound registration attempt. If we already have, we don not try
again until the next attempt. This prevents the infinite loop scenario.

Review: https://reviewboard.asterisk.org/r/4273
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Merged revisions 429761 from http://svn.asterisk.org/svn/asterisk/branches/13

git-svn-id: https://origsvn.digium.com/svn/asterisk/certified/branches/13.1@429869 65c4cc65-6c06-0410-ace0-fbb531ad65f3