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