The "Auto ethX" connection that the system settings service creates
for each wired device that does not have an existing backing connection
provided by one of the system settings plugins is now read/write when
at least one plugin has the MODIFY capability.

When the user updates the "Auto ethX" connection, the system settings
service will try to move that connection to a plugin, thereby preserving
the user's changes.  It will also then save that device's MAC address
and never create an "Auto ethX" connection for it again.