Stop `pterm -ut-' leaving the unnecessary utmp helper as a zombie

process (Debian bug #165887).

[originally from svn r2113]

unix/pty.c

@@ -345,6 +345,7 @@ void pty_pre_init(void)
 	close(pipefd[0]);
 	pty_utmp_helper_pid = pid;
 	pty_utmp_helper_pipe = pipefd[1];
+	signal(SIGCHLD, sigchld_handler);
     }
 #endif