Reentrancy problem if outgoing call gets different B channel than requested.

Reentrancy problem if outgoing call gets different B channel than requested.

The chan_dahdi pri_fixup_principle() routine needs to protect the Asterisk
channel with the channel lock when it changes the technology private
pointer to a new private structure.

* Added lock protection while pri_fixup_principle() moves a call from one
private structure to another.

* Made some pri_fixup_principle() messages more meaningful.

Partial backport from v1.8 -r300714.

git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/1.4@307623 65c4cc65-6c06-0410-ace0-fbb531ad65f3