Create API for fence.register, and use it.

Create API for fence.register, and use it.

This converts the call to minetest.register() for the default
fence node, so it can be called by other mods to quickly
setup other fences.

Since this creates an API, insert it into the game_api.txt.

The api looks like minetest.register(name, {def}), and has two
uncommon fields: "texture" and "material". Any normal nodedef
property can be passed through, except "drawtype". The "fence"
group will always be added.

The default fence recipe is modified to be as follows:
  wood, stick, wood
  wood, stick, wood

This recipe yields 4 fence nodes.

This allows us to create according recipes for acacia, pine,
aspen, and junglewood fences without adding new stick types:

  pine wood, stick, pine wood
  pine wood, stick, pine wood

This is a from-scratch implementation, written by heart but inspired
by (#665 - Add many wooden fences).

Stick and fences nodes are named in a consistent way.