Introduce a check_config phase between pre_config and open_logs,

to allow modules to review interdependent configuration directive
values and adjust them while messages can still be logged to the
console.

The open_logs phase is already used somewhat for this purpose by
certain MPMs (winnt, prefork, worker, and event) but only by forcing
their functions ahead of the core ap_open_logs() function, and
since this phase runs after the ap_signal_server function during startup,
it can not be used to generate messages on the console when restarting.

Add the check_config phase to mod_info and mod_example.

Handle relevant MPM directives during this phase and format messages
for both the console and the error log, as appropriate.  Bounds and sanity
checks on the values of the MPM directives are handled in sequence in
this phase instead of in the various directive handling functions, since
those functions (e.g., set_max_clients()) may not be called at all if their
directives do not appear in the configuration files...