Use default of -1 for uid/gid in class initialization

This doesn't seem to change anything in polkitd behaviour, but it
seems cleaner to default to -1 which here means "unset".

src/polkit/polkitunixgroup.c

@@ -131,9 +131,9 @@ polkit_unix_group_class_init (PolkitUnixGroupClass *klass)
                                    g_param_spec_int ("gid",
                                                      "Group ID",
                                                      "The UNIX group ID",
-                                                     0,
+                                                     -1,
                                                      G_MAXINT,
-                                                     0,
+                                                     -1,
                                                      G_PARAM_CONSTRUCT |
                                                      G_PARAM_READWRITE |
                                                      G_PARAM_STATIC_NAME |

src/polkit/polkitunixuser.c

@@ -144,9 +144,9 @@ polkit_unix_user_class_init (PolkitUnixUserClass *klass)
                                    g_param_spec_int ("uid",
                                                      "User ID",
                                                      "The UNIX user ID",
-                                                     0,
+                                                     -1,
                                                      G_MAXINT,
-                                                     0,
+                                                     -1,
                                                      G_PARAM_CONSTRUCT |
                                                      G_PARAM_READWRITE |
                                                      G_PARAM_STATIC_NAME |