Climbing on one-sided linedefs

Does what it says on the tin. You couldn't before, now you can.

With this branch, thok barriers are now completely optional for maps with no aesthetic need for sky-topped walls. All the bugs and complications that exist with climbing right now still exist, except now there's one less issue making them suck forever and now Nev3r will be very happy.

Do I need a testing .wad? It's... not that hard to create a very small map and go nuts with Knuckles in it. I have one, but uploading it to the ftp almost feels insulting.

See merge request !90