For SIP REGISTER fix domain-only URIs and domain ACL bypass.

The code that allowed admins to create users with domain-only uri's had
stopped to work in 1.8 because of the reqresp parser rewrites. This is
fixed now: if you have a [mydomain.com] sip user, you can register with
useraddr sip:mydomain.com. Note that in that case -- if you're using
domain ACLs (a configured domain list) -- mydomain.com must be in the
allow list as well.

Reviewboard r1606 shows a list of registration combinations and which
SIP response codes are returned.

Review: https://reviewboard.asterisk.org/r/1533/
Reviewed by: Terry Wilson

(closes issue ASTERISK-18389)
(closes issue ASTERISK-18741)
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Merged revisions 346899 from http://svn.asterisk.org/svn/asterisk/branches/1.8
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Merged revisions 346900 from http://svn.asterisk.org/svn/asterisk/branches/10


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@346901 65c4cc65-6c06-0410-ace0-fbb531ad65f3