apparent bug in 4.1BSD DH/DM driver

[utzoo!decvax!watmath!dmmartindale]

The problem with the phantom maintenance-mode receiver interrupt bit staying
set forever is certainly present in our DEC DH11's.  It probably doesn't
occur in the other-vendor DH lookalikes, and since few people actually run
DEC DH's it would be easy for the code to be released with this sort of
bug.  The problem was very obvious with early 4.1BSD releases (which didn't
poll the DH at clock interrupt time at all if you didn't have a Berknet) -
you simply never got receive interrupts.

	Dave Martindale