chan_agent: Misc code cleanup.

* Fix off-nominal path resource cleanup in agent_request().

* Create agent_pvt_destroy() to eliminate inlined versions in many places.

* Pull invariant code out of loop in add_agent().

* Remove redundant module user references in login_exec().

* Remove unused struct agent_pvt logincallerid[] member.
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Merged revisions 378456 from http://svn.asterisk.org/svn/asterisk/branches/1.8
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Merged revisions 378457 from http://svn.asterisk.org/svn/asterisk/branches/11


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@378458 65c4cc65-6c06-0410-ace0-fbb531ad65f3