Convert fences to NDT_CONNECTED.

This changes the drawtype of fences to NDT_CONNECTED nodebox drawtype.

These nodes are drawn by the client with the needed connections on
the fly as the scene is drawn. There is no logic needed by mods to
modify the nodes.

These fences connect to (1) other fences, (2) planks and (3) tree
trunks, but nothing else. They do not connect to stone, dirt, wool,
etc. This is done by the "connects_to" parameter, which takes groups
and node names.

Due to the way textures are wrapped, we can make these nodes look a
lot better by giving them a special tile.

This change requires minetest/minetest#3503.