Add force dial keys to skinny.

Adds a dial softkey when the device is in DAFD. The softkey is greyed (unusable) 
until a possible dialplan match is entered. Code includes updating 
transmit_selectsoftkeys to allow the use of a button mask. Also add option
to use # or * as a dial now button. Original patch by snuffy cleaned up by myself.

Review: https://reviewboard.asterisk.org/r/2277/


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@380057 65c4cc65-6c06-0410-ace0-fbb531ad65f3