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net, internal/poll, net/internal/socktest: use SOCK_{CLOEXEC,NONBLOCK} accept4/socket flags on OpenBSD

The SOCK_CLOEXEC and SOCK_NONBLOCK flags to the socket syscall and the
accept4 syscall are supported since OpenBSD 5.7.

Follows CL 40895 and CL 94295

Change-Id: Icaf35ace2ef5e73279a70d4f1a9fbf3be9371e6c
Reviewed-on: https://go-review.googlesource.com/97196

Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>