conf-files: beef up conf-files.[ch] a bit

This adds fozr new flags:

- If CONF_FILES_DIRECTORY is specified conf_file_list() and friends
  will look for directories only.

- Similar CONF_FILES_REGULAR means we'll look only for regular files.

- If CONF_FILES_BASENAME is specified the resulting list will contain
  only the basenames of all discovered files or directories, not the
  full paths.

- If CONF_FILES_FILTER_MASKED is specified the resulting list will have
  masked entries removed (i.e. those symlinked to /dev/null and
  suchlike)

These four flags are useful for discovering portable service profile
information.

While we are at it, also improve a couple of other things:

- More debug logging

- use path_hash_ops instead of string_hash_ops when putting together the
  path lists

src/basic/exec-util.c

@@ -89,7 +89,7 @@ static int do_execute(
          * If callbacks is nonnull, execution is serial. Otherwise, we default to parallel.
          */
 
-        r = conf_files_list_strv(&paths, NULL, NULL, CONF_FILES_EXECUTABLE, (const char* const*) directories);
+        r = conf_files_list_strv(&paths, NULL, NULL, CONF_FILES_EXECUTABLE|CONF_FILES_REGULAR|CONF_FILES_FILTER_MASKED, (const char* const*) directories);
         if (r < 0)
                 return r;