Add call forward no answer to skinny and cleanup general callfwd handling.

CallforwardNoAnswer uses a sched to determine when to forward the call. 
Defaults to 20secs but configurable in skinny.conf.

Adds dialType to each subchannel structure to be used to differentiate
between normal dials that result in a call being placed (default) and
other uses for the skinny_dialer (such as cfwd digit collection).
Restructured all cfwd handling to use this new arrangement.

(closes issue ASTERISK-21292)
Reported by: wedhorn
Tested by: myself
Patches: 
    skinny-callfwdnoans03.diff uploaded by wedhorn (license 5019)


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@389097 65c4cc65-6c06-0410-ace0-fbb531ad65f3