path-util: return O_DIRECTORY from path_extract_filename() when path ends in slash

path-util: return O_DIRECTORY from path_extract_filename() when path ends in slash

Let's fine-tune the path_extract_filename() interface: on succes return
O_DIRECTORY as indicator that the input path was slash-suffixed, and
regular 0 otherwise. This is useful since in many cases it is useful to
filter out paths that must refer to dirs early on.

I opted for O_DIRECTORY instead of the following other ideas:

1. return -EISDIR: I think the function should return an extracted
   filename even when referring to an obvious dir, so this is not an
   option.

2. S_ISDIR, this was a strong contender, but I think O_DIRECTORY is a
   tiny bit nicer since quite likely we will go on and open the thing,
   maybe with openat(), and hence it's quite nice to be able to OR in
   the return value into the flags argument of openat().

3. A new enum defined with two values "dont-know" and
   "definitely-directory". But I figured this was unnecessary, given we
   have other options too, that reuse existing definitions for very
   similar purposes.