BUG/MEDIUM: mux-h2: Xfer rxbuf to the upper layer when creating a front stream

BUG/MEDIUM: mux-h2: Xfer rxbuf to the upper layer when creating a front stream

Just like the H1 muliplexer, when a new frontend H2 stream is created, the
rxbuf is xferred to the stream at the upper layer.

Originally, it is not a bug fix, but just an api standardization. And in
fact, it fixes a crash when a h2 stream is aborted after the request parsing
but before the first call to process_stream(). It crashes since the commit
8bebd2fe5 ("MEDIUM: http-ana: Don't process partial or empty request
anymore"). It is now totally unexpected to have an HTTP stream without a
valid request. But here the stream is unable to get the request because the
client connection was aborted. Passing it during the stream creation fixes
the bug. But the true problem is that the stream-interfaces are still
relying on the connection state while only the muxes should do so.

This fix is specific for 2.4. No backport needed.