Relinquish exclusive TTY access when tip(1) or cu(1) exit.

Previously they would have left TIOCEXCL enabled, requiring
either a reboot or use of tip/cu as the root user.

Observed when running QEMU with character devices redirected to pty instances.

MFC after:	2 weeks

usr.bin/tip/tip/cmds.c

@@ -666,6 +666,7 @@ tipabort(char *msg)
 	daemon_uid();
 	(void)uu_unlock(uucplock);
 	unraw();
+	unexcl();
 	exit(0);
 }
 

usr.bin/tip/tip/tip.c

@@ -325,6 +325,15 @@ unraw(void)
 		tcsetattr(0, TCSADRAIN, &defterm);
 }
 
+/*
+ * give up exclusive tty access
+ */
+void
+unexcl()
+{
+	ioctl(FD, TIOCNXCL, 0);
+}
+
 static	jmp_buf promptbuf;
 
 /*

usr.bin/tip/tip/tip.h

@@ -341,6 +341,7 @@ void	timeout(int);
 void	tipabort(char *);
 void	tipout(void);
 void	user_uid(void);
+void	unexcl(void);
 void	unraw(void);
 void	v3451_abort(void);
 void	v3451_disconnect(void);

usr.bin/tip/tip/uucplock.c

@@ -126,5 +126,6 @@ uu_unlock(char *ttyname)
 	char tbuf[sizeof(_PATH_LOCKDIRNAME) + MAXNAMLEN];
 
 	(void)snprintf(tbuf, sizeof tbuf, _PATH_LOCKDIRNAME, ttyname);
+	unexcl();
 	return(unlink(tbuf));
 }