Resolve a location to a set of keys in the standard way. If the set is
empty, then the location in INVALID. If the set contains the empty string
(i.e. at least one key.member is "") or the user has access to at least one
of the private keys present in the set, then the location is VALID.
Otherwise, the user does not have one of the private keys listed in the
set, and the location is UNAUTHORIZED.