Fix some potential deadlocks pointed out by helgrind.

Fix some potential deadlocks pointed out by helgrind.

* Fixed deadlock potential calling dialog_unlink_all() in
__sip_autodestruct().  Found by helgrind.

* Fixed deadlock potential in handle_request_invite() after calling
sip_new().  Found by helgrind.

* The sip_new() function now returns with the created channel already
locked.

* Removed the dead code that starts a PBX in in sip_new().  No sip_new()
callers caused that code to be executed and it was a bad thing to do
anyway.

* Removed unused parameters and return value from dialog_unlink_all().

* Made dialog_unlink_all() and __sip_autodestruct() safely obtain the
owner and private channel locks without a deadlock avoidance loop.
........

Merged revisions 340284 from http://svn.asterisk.org/svn/asterisk/branches/1.8

git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/10@340310 65c4cc65-6c06-0410-ace0-fbb531ad65f3