io: reset idcache for new passwd/group (and have a root fallback)

In order to address the problem with early file ownership (before
passwd/group exists), do a few changes here:

1) For root-owned things, always fall back to fixed values, as
   those are the ones we always know and cannot change. Since
   the earliest packages only have root-owned files, this fixes
   the problem for those.
2) During file migration, if we encounter passwd/group and this
   is a newly committed file, reset the idcache, similarly to how
   it is done for scripts. This allows the next package to reload
   the mappings. Since those two files are usually installed as
   a part of the first package, the second package onwards should
   have a valid, complete mapping (reset only by scripts).

0 files changed, 0 insertions, 0 deletions