fs-util: refuse taking a relative path to chase if "root" is specified and CHASE_PREF...

fs-util: refuse taking a relative path to chase if "root" is specified and CHASE_PREFIX_ROOT is set

If we take a relative path we first make it absolute, based on the
current working directory. But if CHASE_PREFIX_ROOT is passe we are
supposed to make the path absolute taking the specified root path into
account, but that makes no sense if we talk about the current working
directory as that is relative to the host's root in any case. Hence,
let's refuse this politely.