inotify support for pbx_spool

This should give a good speed boost, in that one particular thread isn't waking
up once a second to read directory contents.

Reviewboard: https://reviewboard.asterisk.org/r/137/


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@255117 65c4cc65-6c06-0410-ace0-fbb531ad65f3