This allows specific options in pgbackrest.conf to be ignored (and set to default) which reduces the need to write new configuration files for specific needs.
Note that boolean, non-command-line options are already negatable.
When a backup host is present, backups should only be allowed on the backup host and restores should only be allowed on the database host unless an alternate configuration is created that ignores the remote host.
Reported by Lardière Sébastien.
