MINOR: limits: display the computed maxconn using ha_notice()

MINOR: limits: display the computed maxconn using ha_notice()

The computed maxconn was only displayed in verbose or debug modes. This
is too bad because lots of users just don't know what they're starting
with and can be trapped when an environment changes. Let's use ha_notice()
instead of a conditional fprintf() so that it gets displayed right after
the other startup messages, hoping that users will get used to seeing it
and more easily spot anomalies. See github issue #3191 for more context.