[Simh] RP/RM differences in header commands (for ITS salvager)

[Bob Supnik]

Implementing a more complete form of read/write/write check header is 
not as straightforward as I thought, because the RP and RM drives use 
different header formats.

The RP expects 4 16 bit words, of which the first two are used, and the 
second are "for software". Word 0 is cylinder plus 18b/16b, word 1 is 
sector, words 2 and 3 are "keywords".

The RM expects 2 16 bit words, both used. Word 0 is cylinder plus 
18b/16b plus two bad block indicators. Word is 1 is track and sector.

The simulator supports the RP06, RP07, RM02/3, RM05, and RM80. I'm not 
sure if the RP07, RM05, or RM80 were ever actually qualified for the 
KS10. However, the RP06 and RM02 both shipped with the system, so the 2 
word/4 word distinction would have to be implemented.

I can't find the sector format for the RP07 which, despite its name, 
behaved like an RM series drive.

/Bob