plugin_init() works like this:
1. init MyISAM
2. load plugins from mysql.plugin, if it's a MyISAM table
3. init all not initialized plugins
4. all done, if step 2 loaded mysql.plugin,
   otherwise:
5. load plugins from mysql.plugin
6. init all not initialized plugins

now, with --help --verbose, step 3 will not actually
initialize them, and if mysql.plugin is unreadable,
step 6 will try to initialize existing plugins again.
Fix: when skipping initialization because of --help,
change plugin status from PLUGIN_IS_UNINITIALIZED.