Ignore flash-hook when the emergency call ongoing
Check if the outgoing call is going to an emergency number.
If emergency:
set a flag that will be used for chan_voicemngr to ignore any flash-hook during the call, and denied any incoming call with 486 during the call is ongoing.
set a flag for asterisk to add emergency headers.
Should be Merged after this MR from asterisk: asterisk!90 (merged)
Edited by Wenpeng Song