(s)tty: add support for IUTF8 input flag

[bnovkov]

Summary

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.

[imp]

I like this, modulo namespace pollution issue.

sys/sys/_termios.h

103

what standard defines this symbol? I think we'll need some visibility tweaks.

[bnovkov]

sys/sys/_termios.h

103

From what I can tell this is a non-standard symbol that was introduced in Linux at some point, but aside from that the actual symbol name is not set in stone. I'm open to suggestions for different names :D

[christos]

The stty(1) and termios(4) manpages also need to be updated.

[imp]

sys/sys/_termios.h

103

Then ot needs to be hidden behind BSD_VISIBLE

[bnovkov]

Address @imp 's and @christos 's comments.

[bnovkov]

I've updated the manpages for stty and termios and moved IUTF8 behind __BSD_VISIBLE.