MINOR: stats: report the listener's protocol along with the address in stats

MINOR: stats: report the listener's protocol along with the address in stats

When "optioon socket-stats" is used in a frontend, its listeners have
their own stats and will appear in the stats page. And when the stats
page has "stats show-legends", then a tooltip appears on each such
socket with ip:port and ID. The problem is that since QUIC arrived, it
was not possible to distinguish the TCP listeners from the QUIC ones
because no protocol indication was mentioned. Now we add a "proto"
legend there with the protocol name, so we can see "tcp4" or "quic6"
and figure how the socket is bound.