While we do support quic4@ and quic6@ for listening addresses, it was
not possible to specify that we want to use an FD inherited from the
parent with QUIC. It's just a matter of making it possible to enable
a dgram-type socket and a stream-type transport, so let's add this.
Now it becomes possible to write "quic+fd@12", "quic+ipv4@addr" etc.
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