agetty: fix characters reorder in login prompt

agetty: fix characters reorder in login prompt

The current agetty uses TIOCSTI ioctl to return already read chars
from login name back to the terminal (without read() before
tcsetattr() we will lost data already written by user). The ioctl
based solution is fragile due to race -- we can return chars when
terminal already contains another new chars. The result is reordered
chars in login name.

The solution is to use extra buffer for already read data.

Reported-by: Michael Tretter <m.tretter@pengutronix.de>
Signed-off-by: Karel Zak <kzak@redhat.com>