make cobwebs sit in corners when so-placed.

Algorithm tries to find an X-/Z+ or X+/Z- corner first (places the
cobweb using the torchlike "on-floor" mode) then an X-/Z- or X+/Z+
corner ("on-ceiling), then opposing X-/X+ walls (nodebox, not rotated),
then opposing Z-/Z+ walls (nodebox, rotated 90 degrees), then falls back
to plantlike (which also happens to fit places where e.g. three walls
come together only 1m apart)

ABM added to convert old ones.

Node name has changed so if you have cobwebs in your inventory, sorry
about that.