convert-ly rules to destencil Flags

[David Kastrup]

Mike has pushed directly 671b7b63408893c33b4c1f196e87db19a7dbcd1e to
master, as far as I can see without any discussion and without going
through staging.

I changed the rules somewhat in order make them get useful output for
\once\override as well, and pushed this directly to master as a
followup.  Yes, this means that I have not been following procedures as
well, but I think that this is likely to cause the least _additional_
breakage and pain.

Of course, some files in the tree have been fixed _manually_ by Mike
already without adjusting the \version string appropriately, meaning
that running scripts/auxiliar/update-with-convert-ly.sh will rewrite
those files again.

Either a separate commit should be made first that adjusts the version
strings (probably cleanest), or when running convert-ly, the files that
got a second helping of adjustments need to be cleaned up.

Or the convertrules need to be further enhanced in order to recognize
whether they have already been applied previously.  Since this is only
relevant for people not following rules, I think that would be overkill.

In any case, I have about two different changes outstanding that need to
apply their own convertrules.

Until Mike gets his act cleaned up, my changes are stalled since it does
not make sense to commit _his_ convertrules results in the tree as part
of _my_ commits.

-- 
David Kastrup