os_scandir: fname_case -> path_fix_case

Use os_scandir().

fname_case() only gets used when `defined(USE_FNAME_CASE)` (on operating
systems with case-insensitive file systems), but may be useful in other
contexts, so move it to path.c. (See the TODO.)

Remove the unused parameter, len.

src/nvim/buffer.c

@@ -2253,7 +2253,7 @@ setfname (
     }
     sfname = vim_strsave(sfname);
 #ifdef USE_FNAME_CASE
-    fname_case(sfname, 0);            /* set correct case for short file name */
+    path_fix_case(sfname);            /* set correct case for short file name */
 #endif
     free(buf->b_ffname);
     free(buf->b_sfname);