MDEV-9183 MariaDB 10.1 crash on `mysqld --verbose --help`

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.

mysql-test/r/mysqld_help_crash-9183.result

+rename table mysql.plugin to test.plugin;
+flush tables;
+rename table test.plugin to mysql.plugin;

mysql-test/t/mysqld_help_crash-9183.test

+#
+# MDEV-9183 MariaDB 10.1 crash on `mysqld --verbose --help`
+#
+
+rename table mysql.plugin to test.plugin;
+flush tables;
+--exec $MYSQLD_CMD --help --verbose >/dev/null 2>&1
+rename table test.plugin to mysql.plugin;
+

sql/sql_plugin.cc

@@ -1399,6 +1399,7 @@ static int plugin_initialize(MEM_ROOT *tmp_root, struct st_plugin_int *plugin,
   if (options_only || state == PLUGIN_IS_DISABLED)
   {
     ret= 0;
+    state= PLUGIN_IS_DISABLED;
     goto err;
   }