core: add authorization providers and optional PolicyKit support

When the support is complete, use --with-polkit to enable
PolicyKit support.  It's not there yet, but this commit adds an
authorization provider framework which will be extended to allow
hooking into PolicyKit.

src/mm-errors.c

@@ -76,6 +76,7 @@ mm_modem_error_get_type (void)
             ENUM_ENTRY (MM_MODEM_ERROR_DISCONNECTED,            "Disconnected"),
             ENUM_ENTRY (MM_MODEM_ERROR_OPERATION_IN_PROGRESS,   "OperationInProgress"),
             ENUM_ENTRY (MM_MODEM_ERROR_REMOVED,                 "Removed"),
+            ENUM_ENTRY (MM_MODEM_ERROR_AUTHORIZATION_REQUIRED,  "AuthorizationRequired"),
             { 0, 0, 0 }
         };