Extract sig_ss7 out of chan_dahdi.

Extract the SS7 specific code out of chan_dahdi like what was done to
ISDN/PRI and analog signaling.  The new SS7 structures were modeled on
sig_pri.

The changes to sig_pri are an enhancement and a bug fix made possible
because SS7 was extracted.

1) The sig_pri TRANSFERCAPABILITY channel variable should have been set
unconditionally in sig_pri_new_ast_channel().

2) SS7/PRI transfer capability interaction in dahdi_new() fixed because of
SS7 extraction.

3) Module ref count error in dahdi_new() if startpbx failed to start the
PBX for some reason.

Review:	https://reviewboard.asterisk.org/r/661/


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@268774 65c4cc65-6c06-0410-ace0-fbb531ad65f3