Having that extra ID users handling at readfile level, besides generic
one ensured by libquery, has been something bothering me for a long time
(had to fix my share of bugs due to mismatches between those two areas).
Further more, work on undo speedup will require even more complex ID
refcount management if we want to keep it in readfile.c area.
So idea is instead to generalize what we did for linked data already
when undoing: recompute properly usercount numbers after liblink step,
for all IDs.
Note that extra time required here is neglectable in a whole .blend file
reading (few extra milliseconds when loading a full production scene
e.g.).