Prevent an endless loop while executing lxc-attach in the background:

The kernel might fire SIGTTOU while an ioctl() in tcsetattr()
is executed. When the ioctl() is resumed and retries,
the signal handler interrupts it again.

We can't configure the TTY to stop sending
the signals in the first place since that
is a modification/write to the TTY already.

Still we clear the TOSTOP flag to prevent further signals.

Command to reproduce the hang:
----------------------------
cat > lxc_hang.sh << EOF
/usr/bin/timeout 5s /usr/bin/lxc-attach -n SOMECONTAINER -- /bin/true
EOF
sh lxc_hang.sh    # hangs
----------------------------
Signed-off-by: Thomas Jarosch <thomas.jarosch@intra2net.com>