BUG/MINOR: peers: detect and warn on init_addr/resolvers/check/agent-check

BUG/MINOR: peers: detect and warn on init_addr/resolvers/check/agent-check

Some server keywords are currently silently ignored in the peers
section, which is not good because it wastes time on user-side, trying
to make something work while it cannot by design.

With this patch we at least report a few of them (the most common ones),
which are init_addr, resolvers, check, agent-check. Others might follow.

This may be backported to 2.5 to encourage some cleaning of bogus configs.