add plan9 platform

[krzysckh]

This commit adds a Plan9 platform.
This was only tested on 9front.

It doesn't use the font because of a bug in 9front's libttf.

static void
push(Hint *h, u32int w)
{
	assert(h->sp < h->nstack); // ← fails on that assert when trying to load the font
	h->stack[h->sp++] = w;
}

I know adding-as-much-platforms-as-possible is probably not the main idea of this repo, but it demonstrates how easy implementing a platform for the game on a completely different system actually is.

It also messes in a small way with game.c, because 9fronts' pcc (a posix-compatible c compiler) doesn't implement this GNU extension designated initializers in compound literals:

Type thing = (Type) { .foo = 69, .bar = 420 };

so i changed that to

Type thing = (Type) { 69, 420 };

snek.mp4

[krzysckh]

With cdea99d it's now possible to build and run it on unix systems using plan9port for graphics.