BUG/MEDIUM: mux-quic: Don't unblock zero-copy fwding if blocked during nego

BUG/MEDIUM: mux-quic: Don't unblock zero-copy fwding if blocked during nego

The previous fix (792a645ec2 ["BUG/MEDIUM: mux-quic: Unblock zero-copy
forwarding if the txbuf can be released"]) introduced a regression. The
zero-copy data forwarding must only be unblocked if it was blocked by the
producer, after a successful negotiation.

It is important because during a negotiation, the consumer may be blocked
for another reason. Because of the flow control for instance. In that case,
there is not necessarily a TX buffer. And it unexpected to try to release an
unallocated TX buf.

In addition, the same may happen while a TX buf is still in-use. In that
case, it must also not be released. So testing the TX buffer is not the
right solution.

To fix the issue, a new IOBUF flag was added (IOBUF_FL_FF_WANT_ROOM). It
must be set by the producer if it is blocked after a sucessful negotiation
because it needs more room. In that case, we know a buffer was provided by
the consummer. In done_fastfwd() callback function, it is then possible to
safely unblock the zero-copy data forwarding if this flag is set.

This patch must be backported to 3.0 with the commit above.