dahdi_span_config_hook: automatically register new dahdi channels

Install a hook script for DAHDI to register new spans with Asterisk
automatically by running:

  asterisk -rx 'dahdi create channel FIRST LAST'

Review: https://reviewboard.asterisk.org/r/3157/


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@417863 65c4cc65-6c06-0410-ace0-fbb531ad65f3