PDP11: fix RP11 register consistency

Adopt Tony Lawrence's upstream open-simh/simh PR #517 for RP11/RP11-C/-E
register consistency. CPU-visible register reads now mask unimplemented bits,
WLOA protection uses the intended contiguous drive/cylinder range, and detach
cancels pending work in safer order.

Add a PDP-11 script-level regression test for masked RPBA, RPCA, and RPDA bus
reads.

Author: pmetzger