Workaround a problem in recent libedit versions

Reinstall the readline handler when we are called in order to clear the
input line. Tested in a variety of versions by scn and myself.

Fixes #1531

bin/varnishadm/varnishadm.c

@@ -184,6 +184,7 @@ static void send_line(char *l)
 		cli_write(_line_sock, l);
 		cli_write(_line_sock, "\n");
 		add_history(l);
+		rl_callback_handler_install("varnish> ", send_line);
 	} else {
 		RL_EXIT(0);
 	}