Cygwin: Add IUTF8 termios iflag

Cygwin: Add IUTF8 termios iflag

The termios code doesn't handle erasing of multibyte characters
in canonical mode, it always erases a single byte.  When entering
a multibyte character and then pressing VERASE, the input ends up
with an invalid character.

Following Linux we introduce the IUTF8 input flag now, set by
default.  When this flag is set, VERASE or VWERASE will check
if the just erased input byte is a UTF-8 continuation byte.  If
so, it erases another byte and checks again until the entire
UTF-8 character has been removed from the input buffer.

Note that this (just as on Linux) does NOT work with arbitrary
multibyte codesets.  This only works with UTF-8.

For a discussion what happens, see
https://cygwin.com/ml/cygwin/2017-01/msg00299.html

Sidenote: The eat_readahead function is now member of fhandler_termios,
not fhandler_base.  That's necessary to get access to the terminal's
termios flags.

Signed-off-by: Corinna Vinschen <corinna@vinschen.de>