Make it possible to reconstruct config file(s) from I_S.SYSTEM_VARIABLES

Right now there are still several settings that can be set via my.cf that are not visible in SHOW VARIABLES output, or INFORMATION_SCHEMA.SYSTEM_VARIABLES table contents.

As I_S.SYSTEM_VARIABLES contains information on how a variable was set, and in case it was set via "CONFIG" also shows the config file it was set from, it SHOULD be possible to reconstruct the [mariadbd]/[mysqld]/[server] sections of config files by just processing output of

SELECT GLOBAL_VALUE_PATH, VARIABLE_NAME, GLOBAL_VALUE

  FROM INFORMATION_SCHEMA.SYSTEM_VARIABLES

 WHERE GLOBAL_VALUE_PATH IS NOT NULL

 ORDER BY GLOBAL_VALUE_PATH, VARIABLE_NAME

Yet we have several settings, like e.g. master_retry_count, that are not showing in SHOW VARIABLES output or the SYSTEM_VARIABLES table, at all, which makes it basically impossible to determine what they are set to without having file system access to check the actual config files.

Such information MUST be visible to users that only have SQL, but not system/shell access (given they have sufficient SQL level privileges)