MINOR: muxes: Implement ->sctl() callback for muxes and return the stream id

MINOR: muxes: Implement ->sctl() callback for muxes and return the stream id

All muxes now implements the ->sctl() callback function and are able to
return the stream ID. For the PT multiplexer, it is always 0. For the H1
multiplexer it is the request count for the current H1 connection (added for
this purpose). The FCGI, H2 and QUIC muxes, the stream ID is returned.

The stream ID is returned as a signed 64 bits integer.