(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