to support IPv6 only hosts, we try to resolve "localhost" if resolving
"127.0.0.1" fails.

Whatever address we determine is stored in the ${localhost}
macro. Because of the different address:port vs. [address]:port
formats for IPv4 vs. IPv6, we store "${localhost}:0" /
"[${localhost}]:0" in the ${listen_addr} macro.

Likewise we unify the *_sock macros to ${Xaddr}:${Xport} (the colon
used to be missing) for IPv4 and [${Xaddr}]:${Xport} for IPv6.

We also save ${listen_addr} in default_listen_addr for use from within
the vtest code whenever we want to bind to or listen on an emphemeral
port.

Ref #3490