Fix codec capability setup in chan_skinny

Behaviour now is that general codec config flows to default_line and default_device. [devices] stuff amends default_device and similar for [lines]. These are copied to individual device and line as they are created.
Added confcapability and confprefs for the configured stuff which doesn't change as device and so on are connected. prefs are based on line prefs if they exist, else the device prefs are used (prefs identifies codec order).

(closes issue #13806)
Reported by: pj
Patches:
      codecs.diff uploaded by wedhorn (license 30)
Tested by: pj and me


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@163582 65c4cc65-6c06-0410-ace0-fbb531ad65f3