(s)tty: add support for IUTF8 input flag

This patch adds the necessary kernel and stty code to support setting
the IUTF8 flag for ttys. It is the first of two patches that fix
backspace behaviour for UTF-8 encoded characters when in canonical mode.

Reported by:	christos
Reviewed by:	christos, imp
MFC after:	2 weeks
Differential Revision:	https://reviews.freebsd.org/D42066

bin/stty/modes.c

@@ -126,6 +126,8 @@ static const struct modes imodes[] = {
 	{ "-decctlq",	IXANY, 0 },
 	{ "imaxbel",	IMAXBEL, 0 },
 	{ "-imaxbel",	0, IMAXBEL },
+	{ "iutf8",	IUTF8, 0 },
+	{ "-iutf8",	0, IUTF8 },
 	{ NULL,		0, 0 },
 };
 

bin/stty/print.c

@@ -127,6 +127,7 @@ print(struct termios *tp, struct winsize *wp, int ldisc, enum FMT fmt)
 	put("-inpck", INPCK, 0);
 	put("-ignpar", IGNPAR, 0);
 	put("-parmrk", PARMRK, 0);
+	put("-iutf8", IUTF8, 1);
 
 	/* output flags */
 	tmp = tp->c_oflag;

bin/stty/stty.1

@@ -224,6 +224,9 @@ if
 .Cm imaxbel
 is unset and the input queue is full, the next input character causes
 the entire input and output queues to be discarded.
+.It Cm iutf8 Pq Fl iutf8
+Assume that input characters are UTF-8 encoded. Setting this flag
+causes backspace to properly delete multibyte characters in canonical mode.
 .El
 .Ss Output Modes:
 This corresponds to the c_oflag of the termios structure.

share/man/man4/termios.4

@@ -884,6 +884,8 @@ following masks:
 /* any char will restart after stop */
 .It Dv IMAXBEL
 /* ring bell on input queue full */
+.It Dv IUTF8
+/* assume input is UTF-8 encoded */
 .El
 .Pp
 In the context of asynchronous serial data transmission, a break

sys/kern/tty.c

@@ -85,8 +85,8 @@ static const char	*dev_console_filename;
 /*
  * Flags that are supported and stored by this implementation.
  */
-#define TTYSUP_IFLAG	(IGNBRK|BRKINT|IGNPAR|PARMRK|INPCK|ISTRIP|\
-			INLCR|IGNCR|ICRNL|IXON|IXOFF|IXANY|IMAXBEL)
+#define TTYSUP_IFLAG	(IGNBRK|BRKINT|IGNPAR|PARMRK|INPCK|ISTRIP|INLCR|\
+			IGNCR|ICRNL|IXON|IXOFF|IXANY|IMAXBEL|IUTF8)
 #define TTYSUP_OFLAG	(OPOST|ONLCR|TAB3|ONOEOT|OCRNL|ONOCR|ONLRET)
 #define TTYSUP_LFLAG	(ECHOKE|ECHOE|ECHOK|ECHO|ECHONL|ECHOPRT|\
 			ECHOCTL|ISIG|ICANON|ALTWERASE|IEXTEN|TOSTOP|\

sys/sys/_termios.h

@@ -97,6 +97,7 @@
 #endif
 #if __BSD_VISIBLE
 #define	IMAXBEL		0x00002000	/* ring bell on input queue full */
+#define IUTF8		0x00004000	/* assume input is utf-8 encoded */
 #endif
 
 /*