Calling openat(dfd, "/name") ignores the dfd.
Change openat(2) failure expectations to be more specific.

openat(2) failures are generally reported as -ENOTCAPABLE,
not -ECAPMODE.
Remove HAVE_RIGHTS_CHECK_OUTSIDE_CAPMODE

Expect this behaviour everywhere now.
Remove POLLNVAL_FOR_INVALID_POLLFD

Expect this behaviour everywhere now.