Fix worker thread resource leak in SIP TCP/TLS.

Fix worker thread resource leak in SIP TCP/TLS.

The SIP TCP/TLS worker threads were created joinable but noone could join
them if they died on their own.

* Fix the SIP TCP/TLS worker threads to not be created joinable.

* _sip_tcp_helper_thread() only needs one parameter since the pvt
parameter is only passed in as NULL and never used.

(closes issue ASTERISK-19203)
Reported by: Steve Davies

Review: https://reviewboard.asterisk.org/r/1714/
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Merged revisions 356677 from http://svn.asterisk.org/svn/asterisk/branches/1.8

git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/10@356690 65c4cc65-6c06-0410-ace0-fbb531ad65f3