tty: set IUTF8 by default

128f63cedc14 and 9e589b093857 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