HEADS UP: IUTF8 to be enabled by default

Hello again and sorry for the poorly worded previous email,

To give a bit more context, during EuroBSDCon 2023, me and Bojan
Novković started working on a patch to fix backspacing of UTF-8
characters in the tty driver. What was happening is if you typed a >1
byte UTF-8 character and then backspaced it, the driver would actually
delete only 1 byte from the character, instead of all its bytes, which
ended up leaving garbage in the buffer since the character wasn't fully
deleted. To test this, run cat(1), type a UTF-8 character (e.g é, è, à,
non-latin characters, etc), press backspace only once, and look at the
output:

$ cat
αα<backspace>
α�

Bojan then implemented a new IUTF8 flag for stty [1], which enables
proper handling for UTF-8 backspacing in the tty driver [2].

In the Phabricator review of the tty(4) patch [3], I proposed the idea
of having the IUTF8 flag enabled by default. imp@ mentioned that since
the default locale is UTF-8, having the flag set by default shouldn't be
a problem.

Two possible solutions I have thought of:

1. Add IUTF8 to TTYDEF_IFLAG in sys/sys/ttydefaults.h.
2. Add a check in tty_init_termios() whether the current locale is
   UTF-8 (how?), and enable it there.

What do you think? Could this change cause any side-effects we haven't
thought about?

Christos

[1] https://cgit.freebsd.org/src/commit/?id=128f63cedc14ae21b35f74e11e2fe1a5659c58e8
[2] https://cgit.freebsd.org/src/commit/?id=9e589b0938579f3f4d89fa5c051f845bf754184d
[3] https://reviews.freebsd.org/D42067