Refactor bridging and timing to make DAHDI dependency optional

[InterLinked1]

At the time app_rpt was written, DAHDI was the only bridging and timing source. However, nowadays there are multiple timing sources and methods of bridging that are typically more preferred, so being able to utilize those would make DAHDI an optional, rather than required, dependency. This would be a major project but likely make usage easier for folks.

[tsawyer]

This would be a major step forward. DAHDI has been a continuing problem for app_rpt packagers and experimenters.