Add variable length displayprompt packet to skinny and use octals.

Add variable length displayprompt packet to skinny and use octals.

Add new variable length displayprompt packet (0x0145) to skinny. Uses the new
packet if the device is reporting protocol versions >= 17.

Add the use of octal codes for sending prompts to both the new and old
displayprompt messages (also cleaned up soft_key_template_default to use the
defined octal codes).

Review: https://reviewboard.asterisk.org/r/2294/

git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@380792 65c4cc65-6c06-0410-ace0-fbb531ad65f3