Adjust some more to the recent groff 1.20.1 import.

Adjust some more to the recent groff 1.20.1 import.

* Dx is now a built in macro, so remove our local definition. But the
  version supplied with groff still doesn't work correctly due to a
  missing register declaration in doc-common. Fix that locally for now
  (patch sent upstream).

* Sync various definitions (standards, operating systems) with doc-common
  and doc-syms, i.e. remove our local definitions where they are part of
  groff now.