conf-files: include root in returned file paths

This restores the original root handling logic that was present prior to
112cfb18 when path expansion moved to path_strv_canonicalize_absolute.
That behavior partially went away in 12ed81d9.

Alternatively all users of conf_files_list* could be updated to
concatenate the paths themselves as unit_file_query_preset did but since
no user needs the un-concatenated form that is pointless duplication.