'dispvm_allowed' name was confusing, because it suggested being able to
spawn new DispVMs, not being a template for DispVM.
FixesQubesOS/qubes-issues#3047
Those were registered in the parser, but ignored otherwise. The main
(and only?) purpose of them is to evaluate policy when pasting some text
between domains.
FixesQubesOS/qubes-issues#2985
This way it will work independently from where qrexec-policy tool will
be called (in most cases - from a system service, as root).
This is also very similar architecture to what we'll need when moving to
GUI domain - there GUI part will also be separated from policy
evaluation logic.
QubesOS/qubes-issues#910
