[Simh] Various PDP11 fixes and enhancements

[Bob Supnik]

1. RK611 bug - RK07 not recognized on RSTS/E. Fixed. New version 
smoke-tested with VMS, M+, RSTS/E, and RT.
2. RS03/RS04 - third Massbus adapter with RS03/RS04 fixed head disk. 
Only tested with RT11, as M+ and RSTS/E support requires a unique 
SYSGENs. Feel free to hack on it.
3. PDP11 CPU - MMR1 no longer tracks changes to the PC. The issue that 
it should not track changes in floating point instructions is still open.

As usual, look for recent checkins in the GitHub repository, or new 
files in the interim corrections directory on the SimH web site 
(http://simh.trailing-edge.com/interim).

/Bob