The ISDN CPE side should not exclusively pick B channels normally.

The ISDN CPE side should not exclusively pick B channels normally.

Before this patch, Asterisk unconditionally picked B channels exclusively
on the CPE side and normally allowed alternative B channels on the network
side.  Now Asterisk does the opposite.

Reasons for the CPE side to normally not pick B channels exclusively:
*  For CPE point-to-multipoint mode (i.e. phone side), the CPE side does
not have enough information to exclusively pick B channels.  (There may be
other devices on the line.)
*  Q.931 gives preference to the network side picking B channels.
*  Some telcos require the CPE side to not pick B channels exclusively.

(closes issue #14383)
Reported by: mbrancaleoni

git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/1.4@203908 65c4cc65-6c06-0410-ace0-fbb531ad65f3