agetty: use CTRL+C to erase username

agetty: use CTRL+C to erase username

aggety(8) from the beginning ignores ^C (the small exception was
between 2.32 and 2.34 when this char has been misinterpreted).

This patch forces agetty to interpret ^C like ^U, it means to
erase the user's input and wait for a completely new username.
The small difference is that for ^C it does not set 'kill character'.

This change does not affect serial lines where ^C is still ignored like
in previous decades. I'd like to avoid any regression as I have
no clue if any serial lines do not send this control char in some
context ...

Fixes: https://github.com/karelzak/util-linux/issues/1399
References: https://github.com/karelzak/util-linux/issues/1046
Signed-off-by: Karel Zak <kzak@redhat.com>