Add nm_device_connection_compatible() that returns an error when it fails.
nm_device_connection_valid() does the same work except it doesn't set GError.
This commit implements MAC cloning feature in NetworkManager. To support that,
'PermHwAddress' property is added into *.Device.Wired and *.Device.Wireless
interfaces. The permanent MAC address is obtained when creating the device, and
is used for 'locking' connections to the device. If a cloned MAC is specified
in connection to be activated, the MAC is set to the interface in stage1. While
disconecting, the permanent MAC is set back to the interface.
