conf-files: fix an empty root handling in conf_files_list_strv()

conf-files: fix an empty root handling in conf_files_list_strv()

Before 50c81130b69d04288f50217bede709bac6ca2b1a, the function used
chase(), hence if root is an empty string, each config directory made
prefixed with the current working directory if it is relative. See
implementation of chase().

With 50c81130b69d04288f50217bede709bac6ca2b1a, conf_files_list_strv()
internally uses chaseat(), hence each config directory is not prefixed
anymore even if it is relative.

To restore the previous behavior, this makes
- if root is an empty string, prefix each config directories with the
  current working directory if relative.
- if root is relative, make it absolute to make the prefixed results also
  absolute, and debugging logs show absolute paths.
- use chaseat_prefix_root() to prefix the results, for safety.

Follow-ups for 50c81130b69d04288f50217bede709bac6ca2b1a.