bindings: handle current working directory on upgrade

bindings: handle current working directory on upgrade

Recenly I've made failures to run the constructor or destructor fatal.
Mostly, because we can't guarantee correct functionality if they fail.
In fact we can't do anything useful at all (Maybe I can come up with
something more useful soon.)
In any case, this surfaced a bug where on systems that replace/delete
the current workding directory of lxcfs will cause the constructor to
crash on reload because the path that getcwd() returns does not exist
anymore. Fix this by always escaping to root after the construtor has
run but also make it optional in case someone is messing with chroot()s
or other things.

Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>