[Simh] UNIT_RO in simulated units

[Paul Koning]

> On Mar 25, 2018, at 7:23 PM, Bob Supnik <bob at supnik.org> wrote:
> 
> Mark Pizzolato pointed out that UNIT_RO is cleared unconditionally at DETACH. This makes static declaration of a unit as UNIT_RO impossible. He has proposed a change to distinguish a "static" read-only unit (UNIT_RO set but not UNIT_ROABLE) from the usual "dynamic" read-only, set by ATTACH -R or by a unit-specific SET command.

So how does one deal with things like paper tape readers, which are by defition read-only?  I've handled that by marking them ROABLE and supplying the R switch in the attach handler.

	paul