callerid.c: Parse previously ignored Caller ID parameters.

Commit f2f397c1 previously
made it possible to send Caller ID parameters to FXS stations
which, prior to that, could not be sent.

This change is complementary in that we now handle receiving
all these parameters on FXO lines and provide these up to
the dialplan, via chan_dahdi. In particular:

* If a redirecting reason is provided, the channel's redirecting
  reason is set. No redirecting number is set, since there is
  no parameter for this in the Caller ID protocol, but the reason
  can be checked to determine if and why a call was forwarded.
* If the Call Qualifier parameter is received, the Call Qualifier
  variable is set.
* Some comments have been added to explain why some of the code
  is the way it is, to assist other people looking at it.

With this change, Asterisk's Caller ID implementation is now
reasonably complete for both FXS and FXO operation.

Resolves: #681
(cherry picked from commit 44381b2fe90f7e9663a179a2ab27048f994f75d1)