tty: set IUTF8 by default

128f63cedc and 9e589b0938 added proper UTF-8 backspacing handling in
the tty(4) driver, which is enabled by setting the new IUTF8 flag
through stty(1). Since the default locale is UTF-8, and the feature
itself is important enough, enable IUTF8 by default.

Related discussion:
https://lists.freebsd.org/archives/freebsd-arch/2023-November/000534.html

Reviewed by:	imp, bojan.novkovic_fer.hr
Differential Revision:	https://reviews.freebsd.org/D42464

sys/sys/ttydefaults.h

@@ -45,7 +45,7 @@
 /*
  * Defaults on "first" open.
  */
-#define	TTYDEF_IFLAG	(BRKINT	| ICRNL	| IMAXBEL | IXON | IXANY)
+#define	TTYDEF_IFLAG	(BRKINT	| ICRNL	| IMAXBEL | IXON | IXANY | IUTF8)
 #define	TTYDEF_OFLAG	(OPOST | ONLCR)
 #define	TTYDEF_LFLAG_NOECHO (ICANON | ISIG | IEXTEN)
 #define	TTYDEF_LFLAG_ECHO (TTYDEF_LFLAG_NOECHO \