Make all mac-address properties (including NMSettingBluetooth:bdaddr,
NMSettingOlpcMesh:dhcp-anycast-addr, and NMSettingWireless:bssid) be
strings, using _nm_setting_class_transform_property() to handle
translating to/from binary form when dealing with D-Bus.

Update everything accordingly for the change, and also add a test for
transformed setting properties to test-general.