MINOR: mux-h2: report more h2c, last h2s and cs information on "show fd"

MINOR: mux-h2: report more h2c, last h2s and cs information on "show fd"

Most of the time when we issue "show fd" to dump a mux's state, it's
to figure why a transfer is frozen. Connection, stream and conn_stream
states are critical there. And most of the time when this happens there
is a single stream left in the H2 mux, so let's always dump the last
known stream on show fd, as most of the time it will be the one of
interest.