MINOR: config: continue to rely on DEFAULT_MAXCONN to set the minimum maxconn

MINOR: config: continue to rely on DEFAULT_MAXCONN to set the minimum maxconn

Some packages used to rely on DEFAULT_MAXCONN to set the default global
maxconn value to use regardless of the initial ulimit. The recent changes
made the lowest bound set to 100 so that it is compatible with almost any
environment. Now that DEFAULT_MAXCONN is not needed for anything else, we
can use it for the lowest bound set when maxconn is not configured. This
way it retains its original purpose of setting the default maxconn value
eventhough most of the time the effective value will be higher thanks to
the automatic computation based on "ulimit -n".