init-checkconf won't verify .override file

An .override file is simply an additional .conf file for a service providing overrides. It should follow the same format requirements and rules, so it should be possible to troubleshoot/verify with the same script.

When adding the -f option to specify a file, I shouldn't care whether it ends in .conf or .override unless there is a reason a user might not want to copy in a disabled job (without extension) and call it manually, never having it automatically loaded.