Fix glEnablei/glDisablei, update glGet*s for glSampleMaski

I've been semi noting down issues I've found in the manpages while I've been using them, and these are a couple of the ones I noticed most recently. Not having contributed before let me know if there are any glaring errors or structural problems with how I've done things, so I'll know for any more changes.

252170c is a little weird, the change to opengl_spec.py came just from me running read_spec.py - I don't know if those changes are deliberate, but after making the change as you see it glEnablei/glDisablei work fine locally so I presume this is the 'right' way to do redirects :smile:.