Make chan_misdn BRI TE side normally defer channel selection to the NT side.

Make chan_misdn BRI TE side normally defer channel selection to the NT side.

Channel allocation collisions are not handled by chan_misdn very well.
This patch simply avoids the problem for BRI only.

For PRI, allocation collisions are still possible but less likely since
there are simply more channels available and each end could use a different
allocation strategy.

misdn.conf options available:
te_choose_channel - Use to force the TE side to allocate channels.
method - Specify the channel allocation strategy.

(closes issue #13488)
Reported by: Christian_Pinedo
Patches:
      isdn_lib.patch.txt uploaded by crich
Tested by: crich, siepkes, festr

git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/1.4@185120 65c4cc65-6c06-0410-ace0-fbb531ad65f3