Fix ftell on mingw.
* lib/ftell.c (EOVERFLOW): Define if the system lacks it.
* modules/ftell-tests (Depends-on): Add binary-io.
* modules/ftello-tests (Depends-on): Likewise.
* tests/test-ftell.c (main): Enhance test to cover behavior after
ungetc. Enforce binary mode.
* tests/test-ftello.c (main): Likewise.
Signed-off-by: Eric Blake <ebb9@byu.net>