Caminandes team request. In current master it's not possible to do this in a total
clean way, so we are simply setting all user counts of given lib's datablocks to zero
(similar to rna_ID_user_clear()'s doing).
This is a bit crappy because it still lets datablocks floating around (with invalid
user count values), and requires a save & reload cycle to be finalized. But for now
it should be good enough. Propper implementation will be added to id-remap.
842310a9b0