mysql_plugin doesn't read all server option groups

The mysql_plugin utility doesn't read all server option groups from the configuration file. It only reads the [mysqld] option group. This is because it executes the following:

my_print_defaults mysqld

Instead of the following:

my_print_defaults --mysqld

See the following function:

https://github.com/MariaDB/server/blob/3a7d7e180a871c5b2b2d98812d9d7f800cf33952/client/mysql_plugin.c#L314