usernsexec: don't fail if we can't open pty

It's nice if we can do it, but not required.  Exiting on this
failure causes lxc-create started by root as a less-privileged
userid to fail.

Signed-off-by: Serge Hallyn <serge.hallyn@ubuntu.com>
Acked-by: Stéphane Graber <stgraber@ubuntu.com>
introduce lxc-unpriv test

It simply creates a test user and tries to create and start
a container as that user.  Tries to lxc-attach to that
container to test network connectivity.

Signed-off-by: Serge Hallyn <serge.hallyn@ubuntu.com>
Acked-by: Stéphane Graber <stgraber@ubuntu.com>