Support setting and receiving Reverse Charging Indication over ISDN PRI.

This is a continuation of revision 885 to LibPRI (Capture and expose the Reverse
Charging Indication IE on ISDN PRI) which added the ability to get/set Reverse
Charging Indication in LibPRI.  This patch adds the ability to specify RCI on
the outbound leg of a PRI call from within Asterisk, by prefixing the dialed
number with a capital 'C' like:

...,Dial(DAHDI/g1/C4445556666)

And to read it off an inbound channel:

exten => s,1,Set(RCI=${CHANNEL(reversecharge)})

Thanks again to rmudgett for the thorough review.

(closes issue #13760)
Reported by: mrgabu

Review: https://reviewboard.asterisk.org/r/303/


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@204749 65c4cc65-6c06-0410-ace0-fbb531ad65f3