cp,mv,install: improve EACCES targetdir messages

cp,mv,install: improve EACCES targetdir messages

This improves on the fix for --target-directory diagnostics bugs on
Solaris 11.  Problem reported by Bruno Haible and Pádraig Brady; see:
https://lists.gnu.org/r/coreutils/2022-04/msg00044.html
Also, omit some unnecessary stat calls.
* gl/lib/targetdir.c (target_directory_operand): If !O_DIRECTORY,
do not bother calling open if stat failed with errno != EOVERFLOW.
Rename is_a_dir to try_to_open since that’s closer to what it means.
If the open failed with EACCES and we used O_SEARCH, look at stat
results to see whether errno should be ENOTDIR for better diagnostics.
Treat EOVERFLOW as an “I don’t know whether it’s a directory and
there’s no easy way to find out” rather than as an error.