Accept a parked inbound REFER

Accepts an inbound transfer surfaced via leg.transfer_requested (default app-driven model, SIP_REFER_AUTO_DIAL=false). Sends 202 Accepted to the referrer and a NOTIFY sipfrag 100 Trying, keeping the refer subscription open. {id} is the referrer leg (the leg that received the REFER). After accepting, re-bridge as needed and report progress via .../transfer/progress and the outcome via .../transfer/complete.