Prevent deadlock if chan_dahdi attempts to change PRI channel names.

The PRI channels can no longer change the channel name if a different B
channel is selected during call negotiation.  To prevent using the channel
name to infer what B channel a call is using and to avoid name collisions,
the channel name format is changed.

The new channel naming for PRI channels is:
DAHDI/ISDN-<span>-<sequence-number>


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@221701 65c4cc65-6c06-0410-ace0-fbb531ad65f3