Reload configuration from file

Please implement different SQL syntaxes, each will do one of the following:

• Reload the configuration from configuration files, and fail if changes to non-dynamic variables are required.
• Reload the configuration from configuration files, and produce warnings for changes that involve non-dynamic variables.
• Read configuration files, don't make any change, and return variables that will be changed if MariaDB is restarted. For each variable, return whether it's dynamic or not.