chan_sip: keep same local (from) tag for outgoing register requests

For outbound register requests the tag on the From line was
updated every 20 seconds prior to a successful registration
and also once for each registration renewal.  That behavior
can possibly cause the registration to be denied because of
the different tag, and is not aligned with the intention of
RFC 3261 8.1.3.5 "... request constitutes a new transaction
and SHOULD have the same value of the Call-ID, To, and From
of the previous request...".  This updates chan_sip to have
a field to keep the local tag in the registration structure
and use that tag for registration requests where the callid
is also unchanged.

(closes issue ASTERISK-12117)
Reported by: Pawel Pierscionek
Review: https://reviewboard.asterisk.org/r/2988/
........

Merged revisions 402604 from http://svn.asterisk.org/svn/asterisk/branches/1.8


git-svn-id: http://svn.asterisk.org/svn/asterisk/branches/11@402605 f38db490-d61c-443f-a65b-d21fe96a405b