Yet more additions, cmi, szi, sni

[Bill E]

To pdp1.c this time to add some missing pdp-1d instructions. Lia and lai were in already, but for some reason not cmi, complement IO, szi, skip on zero IO, and sni, skip on nonzero IO. There are still a few missing.

[Angelo Papenhoff/aap]

Yeah i didn't implement the PDP-1/D because that's a whole other beast. Due to a lack of software to test with, again. LIA and LAI were needed for the dual-screen version of spacewar, but i *think* this ran on the 1X, which also had them (and probably did so first?). not really sure about the history here, so i just implemented that part of the D which was fairly self-contained. the full D would be quite a bit of surgery :/

[Bill E]

Some of the -1D add-ons are certainly strange, like ring mode. The load/store character instructions and ring mode would be a pain. I think lai, lia, cmi, szi, sni are probably all that should be implemented.

I didn't do any special work for the three added, they are in the OPR section of pdp1.c and come after cli processing. One liners.

Bill