Fix joinable thread terminating without joiner memory leak in chan_iax.c.

The iax2_process_thread() can exit without anyone waiting to join the
thread.  If noone is waiting to join the thread then a large memory leak
occurs.

* Made iax2_process_thread() deatach itself if nobody is waiting to join
the thread.

(closes issue ASTERISK-17339)
Reported by: Tzafrir Cohen
Patches:
      asterisk-1.8.4.4-chan_iax2-detach-thread-on-non-stop-exit.patch (license #5617) patch uploaded by Alex Villacis Lasso (modified)

(closes issue ASTERISK-17825)
Reported by: wangjin
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Merged revisions 350220 from http://svn.asterisk.org/svn/asterisk/branches/1.8
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Merged revisions 350221 from http://svn.asterisk.org/svn/asterisk/branches/10


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@350222 65c4cc65-6c06-0410-ace0-fbb531ad65f3