Dialing an invalid extension causes incomplete hangup sequence.

Revision -r1489 of the libpri 1.4 branch corrected a deviation from Q.931
Section 5.3.2.  However, this resulted in an unexpected behaviour change
to the upper layer (Asterisk).

This change uses pri_hangup_fix_enable() to follow Q.931 Section 5.3.2
call hangup better if the version of libpri supports it.

(issue #17104)
Reported by: shawkris
Tested by: rmudgett


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@262569 65c4cc65-6c06-0410-ace0-fbb531ad65f3