The dialing flag was mistakingly removed from sig_pri.

This readds the proper setting of the flag and is really a continuation of
r205731. The flag was being set properly in sig_analog, but use of the 
newly added set_dialing callback allowed for some simplification in
chan_dahdi.

(closes issue #15486)
Reported by: rmudgett


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@206767 65c4cc65-6c06-0410-ace0-fbb531ad65f3