syscall: hostname/domainname fix for openbsd

Work around a bug that was fixed after OpenBSD 5.0 - a request for
kern.hostname or kern.domainname with a nil value for oldp will result
in a length of zero being returned. If we hit this case use a length
of MAXHOSTNAMELEN (256).

R=golang-dev, mikioh.mikioh
CC=golang-dev
https://golang.org/cl/5408041

src/pkg/syscall/syscall_bsd.go

@@ -559,8 +559,17 @@ func Sysctl(name string) (value string, err error) {
 		return "", err
 	}
 	if n == 0 {
+		// TODO(jsing): Remove after OpenBSD 5.2 release.
+		// Work around a bug that was fixed after OpenBSD 5.0.
+		// The length for kern.hostname and kern.domainname is always
+		// returned as 0 when a nil value is passed for oldp.
+		if OS == "openbsd" && (value == "kern.hostname" || value == "kern.domainname") {
+			// MAXHOSTNAMELEN
+			n = 256
+		} else {
 			return "", nil
 		}
+	}
 
 	// Read into buffer of that size.
 	buf := make([]byte, n)