Change the behaviour of DTMF sending

When both "use_rfc2833" and "use_info" are enabled on a Core, use SIP INFO only as fallback for when a media does not support out-of-band DTMF events

Parent: flexisip!742 (merged)

Children:

• bctoolbox!199 (merged)
• mediastreamer2!720 (merged)
• liblinphone!2227 (merged)