added picket, white picket, and barbed wire corner fences, and recipes to make
them.

Tweaked picket fence nodeboxes slightly to push them as close to the
back of the node as I could without Z-fighting with neighboring objects.

Minor code formatting tweaks

got rid of some commented-out code.
"top" edges of picket fences/gates should be transparent, since the
fences are only 2d anyway.
Add corner privacy fence and craft recipes for it.
minor tweaks to corner fence selection boxes
Several changes (see below)

Dropped support for punching doors to open them - only right-click now.

Moved gates out of fences and into doors file, renamed that file to
doors_and_gates.lua.

Changed gates to also use right click to open, and in the process
completely rewrote how gates are defined and managed; this necessitated
creating some duplicate textures.  Some of them are blank, and texture
pack authors may find the extra filenames useful anyway.

Node names for all gates have changed, aliases are provided for backward
compatibility.
fixed a minor texture bug in corner chainlink fences/