core: use same codepaths for root and non-root during authentication

Instead of doing something like

<get caller UID>
if (root) {
   perform_operation()
   other boilerplate stuff
   return;
}

nm_auth_chain_new(perform_operation)
...

just have root also go through the auth chain, which is now
short circuited for root.  This ensures we always use the same
code paths for root and non-root, and that fixes made in one path
are also executed for the other.

src/nm-manager-auth.h

@@ -57,14 +57,17 @@ typedef void (*NMAuthChainResultFunc) (NMAuthChain *chain,
 
 NMAuthChain *nm_auth_chain_new (DBusGMethodInvocation *context,
                                 DBusGProxy *proxy,
+                                gulong user_uid,
                                 NMAuthChainResultFunc done_func,
                                 gpointer user_data);
 
 NMAuthChain *nm_auth_chain_new_raw_message (DBusMessage *message,
+                                            gulong user_uid,
                                             NMAuthChainResultFunc done_func,
                                             gpointer user_data);
 
 NMAuthChain *nm_auth_chain_new_dbus_sender (const char *dbus_sender,
+                                            gulong user_uid,
                                             NMAuthChainResultFunc done_func,
                                             gpointer user_data);