Dump "mysql" system database first on "mysqldump --all-databases"

When doing mysqldump --all-databases, databases are simply dumped in alphabetical order.

I'd like to suggest to make an exception for the "mysql" system database, dumping that first, and emitting a FLUSH PRIVILEGES to the dump stream right after that, before proceeding to dump all user defined databases.

That way it would e.g. become possible to restore dumps that contain FederatedX tables that refer to specific mysql.servers entries, which is otherwise not possible with mysqldump yet as we are missing SHOW CREATE SERVER and SHOW SERVERS equivalents to CREATE SERVER ....