Revert "modules: fall-back to system dlopen search path."

The original patch had security issues, and broke loading of
frozen files including modules that are only available along
the fall-back path.

This reverts commit 1d887c9b5002d8e9a4e65c192c8a1b7355cc356a.
modules: support 8.3 truncated filenames.

* m4/path.c (TRUNCATE_FILENAME): New macro, defined on OS2.
(path_truncate): New function when TRUNCATE_FILENAME is defined,
otherwise a null-operation macro.  Trim the basename to no more
than 8 characters, followed by the extension.
(m4_path_search): Use it.
Reported by Ko Myung-Hun

Signed-off-by: Gary V. Vaughan <gary@gnu.org>