Cleaned up chan_dahdi iflist handling and locking.

*  Fixed walking the iflist so it is always done with the iflock locked.
*  Simplified iflist walking routines.
*  Created chan_dahdi iflist insertion and extraction routines.
*  Fixed duplicate_pseudo() malloc fail handling.
*  Fixed infinite loop in action_dahdishowchannels() when showing a single channel.


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@217912 65c4cc65-6c06-0410-ace0-fbb531ad65f3