Fix a few problems... No plugin should return secrets in the GetSettings method,
which some plugins did. When that was committed in the commit "system-settings:
don't return secrets in the settings", it broke those plugins that didn't implement
GetSecrets. Each plugin can actually use the same code for GetSettings and
GetSecrets, so implement those generically in the NMExportedConnection class and
remove plugin-specific implementations that all did the same thing.
Clean up handling of "special" keys in keyfiles, ie ones that
need more processing than the basic GKeyFile API supports. Add
MAC address reading (writing support to come).
Additionally, add some test bits for the keyfile plugin that get
run on 'make check'.
