unix: add GetsockoptString for Darwin, *BSD and Solaris

As suggested by Brad in CL 83275.

From what I can tell getsockopt syscall on these OSes doesn't return
ERANGE in any case, so these versions don't include the buffer growing.

Change-Id: I667c3751021e26defec0a04761a689fa4b78c65d
Reviewed-on: https://go-review.googlesource.com/84016
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

unix/syscall_solaris.go

@@ -130,6 +130,18 @@ func Getsockname(fd int) (sa Sockaddr, err error) {
 	return anyToSockaddr(&rsa)
 }
 
+// GetsockoptString returns the string value of the socket option opt for the
+// socket associated with fd at the given socket level.
+func GetsockoptString(fd, level, opt int) (string, error) {
+	buf := make([]byte, 256)
+	vallen := _Socklen(len(buf))
+	err := getsockopt(fd, level, opt, unsafe.Pointer(&buf[0]), &vallen)
+	if err != nil {
+		return "", err
+	}
+	return string(buf[:vallen-1]), nil
+}
+
 const ImplementsGetwd = true
 
 //sys	Getcwd(buf []byte) (n int, err error)