MINOR: listener: set the QUIC xprt layer immediately after parsing the args

MINOR: listener: set the QUIC xprt layer immediately after parsing the args

It used to be set when parsing the listeners' addresses but this comes
with some difficulties in that other places have to be careful not to
replace it (e.g. the "ssl" keyword parser).

Now we know what protocols a bind_conf line relies on, we can set it
after having parsed the whole line.