MINOR: Add sample fetches to get the frontend and backend stream ID

MINOR: Add sample fetches to get the frontend and backend stream ID

"fc.id" and "bc.id" sample fetches can now be used to get, respectively, the
frontend or the backend stream ID. They rely on ->sctl() callback function
on the mux attached to the corresponding SC.

It means these sample fetches work only for connection, not applets, and
from the time a multiplexer is installed.