MINOR: extcheck: fill in the server's UNIX socket address when known

MINOR: extcheck: fill in the server's UNIX socket address when known

While it's often a pain to try to figure a UNIX socket address, the
server ones are reliable and may be emitted in the check provided
they are retrieved in time. We cannot rely on addr_to_str() because
it only reports "unix" since it may be used to log client addresses
or listener addresses (which are renamed).

The address length was extended to 256 chars to deal with long paths
as previously it was limited to INET6_ADDRSTRLEN+1.

This addresses github issue #101. There's no point backporting this,
external checks are almost never used.