Fix#11520andsomeotherimprovements

Tries to fix #11520

Also, some improvements in the related code in a separate commit.
Previously, we were not allowing to create users with user-added auth plugins, now this is allowed.

As this involves many different versions of MySQL as well as MariaDb, please report if you get any failures in any operation, due to this commit.

该提问来源于开源项目：phpmyadmin/phpmyadmin