[MINOR] try to guess server check port when unset

[MINOR] try to guess server check port when unset

When a server has no port specified and there is a check
enabled on it, the check is disabled because the port is
unknown. However, people expect the "listen" line to set
the check port just like it sets the server's port. Now,
if a port is specified in the listen or in the first bind
and nowhere else, it will be used for the checks as well.