settings: port NMAgentManager, etc, to use NMAuthSubject

Rather than explicitly passing around a UID and a flag saying whether
or not it's relevant.

(This also fixes a bug where the wrong UID was being recorded in
nm-settings-connection.c::auth_start(), which caused problems such as
agent-owned secrets not getting saved because of a perceived UID
mismatch.)

src/nm-active-connection.c

@@ -282,17 +282,6 @@ nm_active_connection_get_user_requested (NMActiveConnection *self)
 	return !nm_auth_subject_get_internal (NM_ACTIVE_CONNECTION_GET_PRIVATE (self)->subject);
 }
 
-gulong
-nm_active_connection_get_user_uid (NMActiveConnection *self)
-{
-	NMActiveConnectionPrivate *priv;
-
-	g_return_val_if_fail (NM_IS_ACTIVE_CONNECTION (self), G_MAXULONG);
-	priv = NM_ACTIVE_CONNECTION_GET_PRIVATE (self);
-
-	return nm_auth_subject_get_uid (priv->subject);
-}
-
 NMDevice *
 nm_active_connection_get_device (NMActiveConnection *self)
 {