BUG/MEDIUM: mux-h2: Make sure we remove H2_SF_NOTIFIED on subscribe

BUG/MEDIUM: mux-h2: Make sure we remove H2_SF_NOTIFIED on subscribe

The mux h2 remembers that he woke a stream for writing, and as long as
the H2_SF_NOTIFIED flag is set, won't attempt to do so again. However,
if for any reason the stream does not try to send data, and just
subscribes, then it will never be woken up. So remove the flag on
subscribe.
This can happen for example, with tcpcheck using the proxy protocol, if
for any reason the proxy protocol handshake is not done yet, as tcpcheck
will just subscribe if it sees the CO_FL_WAIT_XPRT flag.

This should be backported up to 2.8.