This allows implementing some convenience features in nmcli -- listing
the backing store for the connection in "nmcli c show", and using the
filename for specifying connection in "nmcli c up/down".

Eventually, paired with ReloadConnections(), this could be used to
implement something similar to what "systemctl edit" does for units
(though we'd need to pick another command name as we aready use
"nmcli c edit" for something different).