nonblocking: fix mingw test failures

nonblocking: fix mingw test failures

Actually testing on mingw uncovered a few more problems.

* lib/nonblocking.c (set_nonblocking_flag): Succeed when clearing
non-blocking flag on regular file.
(get_nonblocking_flag): Set errno on invalid fd.
* tests/test-nonblocking.c (main): Avoid test failure on
directories if fchdir is not active.
* modules/nonblocking-tests (Depends-on): Drop unused dependency.

Signed-off-by: Eric Blake <eblake@redhat.com>