Don't reset the activity timer for unattached sessions every second,

this screws up the choice of most-recently-used. Instead, break the time
update into a little function and do it when the session is attached.

Pointed out by joshe@.

cmd-switch-client.c

@@ -173,6 +173,7 @@ cmd_switch_client_exec(struct cmd *self, struct cmd_ctx *ctx)
 	if (c->session != NULL)
 		c->last_session = c->session;
 	c->session = s;
+	session_update_activity(s);
 
 	recalculate_sizes();
 	server_check_unattached();