Change pm.start_servers default warning to notice

Unlike other settings, this has a perfectly reasonable default, calculated using
a dynamic formula. If the default was hardcoded to "2" or something, then it
would make sense to have a warning, since that could potentially be bad, but for
a dynamically calculated value based on other mandatory settings, a notice ought
to be enough.
move zlog_set_level() again

from fpm_unix_init_main() to fpm_conf_post_process() this time (see #894),
because otherwise nothing in fpm_conf_init_main() obeys log levels
NEWS