MINOR: hq-interop: properly handle incomplete request

MINOR: hq-interop: properly handle incomplete request

Extends HTTP/0.9 layer to be able to deal with incomplete requests.
Instead of an error, 0 is returned. Thus, instead of a stream closure.
QUIC-MUX may retry rcv_buf operation later if more data is received,
similarly to HTTP/3 layer.

Note that HTTP/0.9 is only used for testing and interop purpose. As
such, this limitation is not considered as a bug. It is probably not
worth to backport it.