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>