agetty: keep c_iflags unmodified on --autologin

agetty: keep c_iflags unmodified on --autologin

agetty sets c_iflags according to interaction with serial line in
get_logname(). For --autologin it does not read from the line, so we
have no clue how to set the flags.

The current behavior is to zeroize the flags.  Unfortunately, it seems
like bad idea, because the line may be already properly initialized by
kernel (or systemd, etc.).

The new behavior is not touch the flags on --autologin.

Addresses: https://bugzilla.redhat.com/show_bug.cgi?id=1252764
Signed-off-by: Karel Zak <kzak@redhat.com>