MEDIUM: h3: prevent new streams on GOAWAY reception

MEDIUM: h3: prevent new streams on GOAWAY reception

Implement the reception of a HTTP/3 GOAWAY frame. This is performed via
the new function h3_parse_goaway_frm(). The advertised ID is stored in
new <id_shut_r> h3c member. It serves to ensure that a bigger ID is not
advertised when receiving multiple GOAWAY frames.

GOAWAY frame reception is only really useful on the backend side for
haproxy. When this occurs, h3c is now flagged with H3_CF_GOAWAY_RECV.
Also, QCC is also updated with new flag QC_CF_CONN_SHUT. This flag
indicates that no new stream may be opened on the connection. Callback
avail_streams() is thus edited to report 0 in this case.