Write warning in error log if option is set in configuration files multiple times

When a user's configuration file has an !includedir directive and the user has accidentally set an option multiple times in multiple files, a lot of the time the user doesn't even notice until things aren't working as they expect them to. This is more often true for users who use our enterprise packages, since mariadb-enterprise.cnf sets a bunch of common options.

If an option is set multiple times in configuration files, it would be great if some kind of warning similar to the following could be printed to the error log:

[Warning] Option 'binlog_format' set multiple times in configuration files. Using the last value read from file 'z.cnf'. Using value 'MIXED'.