Allow res_calendar to be unloaded

The calendaring tech modules depend on res_calendar and initially
res_calendar just bumped the use count so that it couldn't be unloaded.
res_calendar can potentially create many threads and I've seen issues
where the Asterisk shutdown has failed where it looked like these
threads could be the culprit.

This patch adds unload support for res_calendar. Unloading res_calendar
will also unload the dependant tech modules as well.

(closes issue ASTERISK-16744)
Review: https://reviewboard.asterisk.org/r/1657/
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Merged revisions 353502 from http://svn.asterisk.org/svn/asterisk/branches/1.8
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Merged revisions 353503 from http://svn.asterisk.org/svn/asterisk/branches/10


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@353504 65c4cc65-6c06-0410-ace0-fbb531ad65f3