MEDIUM: sock-inet: re-check IPv6 connectivity every 30s

MEDIUM: sock-inet: re-check IPv6 connectivity every 30s

IPv6 connectivity might start off (e.g. network not fully up when
haproxy starts), so for features like resolvers, it would be nice to
periodically recheck.

With this change, instead of having the resolvers code rely on a variable
indicating connectivity, it will now call a function that will check for
how long a connectivity check hasn't been run, and will perform a new one
if needed. The age was set to 30s which seems reasonable considering that
the DNS will cache results anyway. There's no saving in spacing it more
since the syscall is very check (just a connect() without any packet being
emitted).

The variables remain exported so that we could present them in show info
or anywhere else.

This way, "dns-accept-family auto" will now stay up to date. Warning
though, it does perform some caching so even with a refreshed IPv6
connectivity, an older record may be returned anyway.