Ignore the user of the system [mysqld] config

[upekkha]

Description

This is related to issue #897.

Any local user configuration gets ignored and overwritten by a user defined in the [mysqld] section of the global config. For instance Debian/Ubuntu are affected, as the /etc/mysql/mariadb.conf.d/50-server.conf constains user = mysql. Thus mycli fails to connect,

# mycli
Connecting to socket /var/run/mysqld/mysqld.sock, owned by user mysql
(1698, "Access denied for user 'mysql'@'localhost'")

unless the user is manually specified with mycli -u root. In previous versions this was not the case.

This PR extends the previous #968 and also adds user to the key_transformations dict, so that it gets ignored.

Checklist

• [X] I've added this contribution to the changelog.md.
• [X] I've added my name to the AUTHORS file (or it's already there).