There are a lot of niggling fixes, mostly for incompatibility issues
found by Walter Mueller. Working out the 11/70 behavior is still not
fully done, but it's a lot better than it was, and it passes more of the
11/70 MMU diagnostic.
- Floating point. Now model sensitive in treating MMR1, for 2.11 BSD.
- MMR1 now tracks PC changes on 11/44, 11/45, 11/70, and J11 -(PC) and @-(PC).
- MMR1 is cleared at start of trap sequence on 11/45, 11/70.
- Red stack abort occurs before memory writes on 11/45, 11/70 (no other model has them).