To make sure curl doesn't allow multiplexing before a connection is
upgraded to HTTP/2 (like when Upgrade: h2c fails), we must make sure the
connection uses HTTP/2 as well and not only check what's wanted.
When an HTTP/2 upgrade request fails (no protocol switch), it would
previously detect that as still possible to pipeline on (which is
acorrect) and do that when PIPEWAIT was enabled even if pipelining was
not explictily enabled.
It should only pipelined if explicitly asked to.