Add autosave/restore hook system

[curiousdannii]

Use the same hook system as in Glulxe.

[curiousdannii]

@erkyrath I've been thinking about how to go about this, and I think it's going to be even more messy than normal :)

GlkOte wants to call do_autosave(), but I don't think that will work here. Emglken has to turn the synchronous glk_select() into an asynchronous one, by pausing and serialising the stack. I don't think I could get do_autosave() to call more C code (to produce a savefile for example). So I think I will make do_autosave() be almost a noop (except for clearing the autosave if it's given -1).

So to do the autosaving I think I will do that inside glk_select(). Should that work?

Actually that might be a bad idea, because for a terp like Git, it's glkop.c already tries to run the autosave hook... so maybe I'll have to localise it to Hugo's code whenever it calls glk_select().