lxc-attach: fix redirection - stderr

lxc-attach: fix redirection - stderr

So that we can do things like:

	lxc-attach -n a -- sh -c 'echo ERR >&2' > /dev/null

There seems to be no easy way to discern when we need to write to stderr
instead of stdout when we receive an event on the master fd of an allocated
pty. So we're using a "trick"/"hack". We write to STDOUT_FILENO if it refers to
a pty. If STDOUT_FILENO does not refer to a pty we check whether STDERR_FILENO
refers to a pty and if so write to it.

Signed-off-by: Christian Brauner <christian.brauner@mailbox.org>