Add an explicit method for forcing emulated VGA output. Previously it
was possible only by removing `gui` feature (setting it to false had a
different effect), or enabling debug mode.
Using lack of a feature as a third state was a bad idea.
QubesOS/qubes-issues#3585
Most of them are moved from core-admin repository, even if tool is not
implemented yet. Some are generated using skel-manpage.py script.
This way we have complete man pages list, and sphinx will properly
warn/error about inconsistencies.
