The test binary has two modes: in the default argument-less mode, it
just checks that "root" can be resolved. When invoked manually, a root
prefix and user/group names can be specified.
There are two libc APIs for accessing the user database: NSS/getpwuid(),
and fgetpwent(). if we run in --root= mode (i.e. "offline" mode), let's
use the latter. Otherwise the former. This means tmpfiles can use the
database included in the root environment for chowning, which is a lot
more appropriate.
Fixes: #14806
