path-util: tighten path_extract_filename()

path-util: tighten path_extract_filename()

Let's tighten the logic behind path_extract_filename() a bit: first of
all, refuse all cases of invalid paths with -EINVAL. More importantly
though return a recognizable error when a valid path is specified that
does not contain any filename. Specifically, "/" will now result in
-EADDRNOTAVAIL.

This changes API, but none of the existing callers care about the return
value, hence the change should be fine.