mysql_upgrade should check for unavailable plugin libraries

Right now mysql_upgrade does not check whether installed plugins can actually be loaded.

This is especially an issue when upgrading from MySQL to MariaDB, where e.g. validate_password.so might be installed which does not exist in MariaDB.

Right now mysql_upgrade does only check for the presence of needed storage engine plugins, all other missing plugins will only be noticed on server startup and error messages will be written to the error log, where they might get overlooked.