MINOR: listener: automatically select a QUIC mux with a QUIC transport

MINOR: listener: automatically select a QUIC mux with a QUIC transport

When no mux protocol is configured on a bind line with "proto", and the
transport layer is QUIC, right now mux_h1 is being used, leading to a
crash.

Now when the transport layer of the bind line is already known as being
QUIC, let's automatically try to configure the QUIC mux, so that users
do not have to enter "proto quic" all the time while it's the only
supported option. this means that the following line now works:

    bind quic4@:4449 ssl crt rsa+dh2048.pem alpn h3 allow-0rtt