openbsd: fix poll(2) with setrlmit(RLIMIT_NOFILE)

Like Linux and Solaris, poll(2) on OpenBSD will return EINVAL if the
number of file descriptors argument passed to poll is above the maximum
number of fd's allowed by setrlimit(2).

By inspection, NetBSD behaves in a similar way to FreeBSD (nfd is
allowed to exceed maximum fd).