[Simh] Read/write header in RP drives

[Bob Supnik]

I like Johnny's suggestion.

1. On write header, "eat" the two header words and then write normal data.
2. On read header, synthesize the two standard header words and then 
read the pack data.
3. On write check header, skip the first two (header) words and then do 
a normal write check.

This is what the DECtape simulators do with Read All and Write All. This 
solution would work on the PDP11/VAX RPs as well.

I also agree that the "white space" could be used to record the header 
information for 36b drives, but it would be safer to leave those bits as 
zeroes. If the upper 28b of simulated PDP10 memory ever held non-zero 
data, the CPU simulator would go off the rails, badly. And the solution 
would be unique to the PDP10 implementation.

/Bob