Set FPM log level earlier during init

The log level will be ignored throughout log events in fpm_unix_init_main, until
the very end when zlog_set_level is finally called.

In particular, fpm_unix_conf_wp may throw notices like this:

> [08-Nov-2014 17:13:36] NOTICE: [pool www] 'user' directive is ignored when…

Which will still appear even if log_level in php-fpm.conf is set to warning+