borg2: implement "real" users/groups (windows)

on linux, borg archives uid/gid and user/group from `stat_result.st_uid/gid` and also restores that accordingly (given it has the permissions). at some places, borg uses uid/gid 0 or expects uid/gid 0 to resolve to something (like "root").

on windows, this is obviously a bit different. to avoid breakage all over the place, borg and the tests currently use a quick hack in `borg.platform.windows` so that always just returns 0 and "root".