tty: enable the echoing of ^C in the N_TTY discipline

Turn on INTR/QUIT/SUSP echoing in the N_TTY line discipline (e.g.  ctrl-C
will appear as "^C" if stty echoctl is set and ctrl-C is set as INTR).

Linux seems to be the only unix-like OS (recently I've verified this on
Solaris, BSD, and Mac OS X) that does *not* behave this way, and I really
miss this as a good visual confirmation of the interrupt of a program in
the console or xterm.  I remember this fondly from many Unixs I've used
over the years as well.  Bringing this to Linux also seems like a good way
to make it yet more compliant with standard unix-like behavior.

[akpm@linux-foundation.org: coding-style fixes]
Cc: Alan Cox <alan@lxorguk.ukuu.org.uk>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

drivers/char/n_tty.c

@@ -769,7 +769,21 @@ static inline void n_tty_receive_char(struct tty_struct *tty, unsigned char c)
 		signal = SIGTSTP;
 		if (c == SUSP_CHAR(tty)) {
 send_signal:
-			isig(signal, tty, 0);
+			/*
+			 * Echo character, and then send the signal.
+			 * Note that we do not use isig() here because we want
+			 * the order to be:
+			 * 1) flush, 2) echo, 3) signal
+			 */
+			if (!L_NOFLSH(tty)) {
+				n_tty_flush_buffer(tty);
+				if (tty->driver->flush_buffer)
+					tty->driver->flush_buffer(tty);
+			}
+			if (L_ECHO(tty))
+				echo_char(c, tty);
+			if (tty->pgrp)
+				kill_pgrp(tty->pgrp, signal, 1);
 			return;
 		}
 	}