net: don't set wrong option for controlling tcp keepalive on openbsd

Fixes golang#8679.

LGTM=iant
R=golang-codereviews, iant
CC=golang-codereviews
https://golang.org/cl/141730043

src/net/tcpsockopt_openbsd.go

@@ -2,26 +2,15 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-// TCP socket options for openbsd
-
 package net
 
 import (
-	"os"
 	"syscall"
 	"time"
 )
 
-// Set keep alive period.
 func setKeepAlivePeriod(fd *netFD, d time.Duration) error {
-	if err := fd.incref(); err != nil {
-		return err
-	}
-	defer fd.decref()
-
-	// The kernel expects seconds so round to next highest second.
-	d += (time.Second - time.Nanosecond)
-	secs := int(d.Seconds())
-
-	return os.NewSyscallError("setsockopt", syscall.SetsockoptInt(fd.sysfd, syscall.IPPROTO_TCP, syscall.SO_KEEPALIVE, secs))
+	// OpenBSD has no user-settable per-socket TCP keepalive
+	// options.
+	return syscall.EPROTONOSUPPORT
 }