[Simh] More 18b bits and bobs

[Bob Supnik]

The RM09 has the same size options and programming model as the Type 24 
serial drum on the PDP4 and PDP9. Perhaps the "UD" option referenced in 
the services list was a later, larger capacity model, but there's no 
documentation one way or another.

It's very easy to add the drum to the PDP-9, and I'll do that along with 
the other updates I mentioned in previous messages.

/Bob