agetty: don't use log_err() for non-fatal errors

agetty: don't use log_err() for non-fatal errors

The TIOCSCTTY ioctl requires that caller is session leader -- so it
depends on initd (or we have to add setsid() to aggety). It seems that the
traditional way is to setup tty in agetty and session in login(1).

It means that all session related things (TIOCSCTTY, vhangup, ...) in the
command agetty should be optional. (Note that vhangup() is called when
--hangup is explicitly specified on command line, so log_err() makes
sense there.)

Reported-by: Andrew Walrond <andrew@walrond.org>
Signed-off-by: Karel Zak <kzak@redhat.com>