Implement watching of nodes which don't exist.

Implement watching of nodes which don't exist.
Requires permission check every time event is generated.
Requires generalization of permissions: ask arbitrary number of
parents whether it's OK to tell about node (eg. watching
/dir/subdir/x when /dir is deleted: root permissions will now
determine whether we fire event).
Add test that we don't leak information on whether a file exists
or not.
Signed-off-by: Rusty Russel <rusty@rustcorp.com.au>
Signed-off-by: Christian Limpach <Christian.Limpach@cl.cam.ac.uk>