unix: add IoctlGetInt and IoctlSetInt on Linux

This CL adds basic integer get/set functions which wrap a generic
ioctl call.  The API is similar the one introduced for solaris/amd64
in CL 14587, but the request parameter has been changed to a uint
instead of an int.  This makes requests with a number larger than the
maximum signed 32-bit integer work on linux/386.

For consistency, the solaris/amd64 API has also been updated to make
use of a uint instead of an int for the request number.

Fixes golang/go#20474

Change-Id: Iaae1ee2e4bb4bfcc420dcec252fe53c8d90ce81d
Reviewed-on: https://go-review.googlesource.com/44009
Run-TryBot: Matt Layher <mdlayher@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

unix/syscall_linux_test.go

@@ -15,6 +15,21 @@ import (
 	"golang.org/x/sys/unix"
 )
 
+func TestIoctlGetInt(t *testing.T) {
+	f, err := os.Open("/dev/random")
+	if err != nil {
+		t.Fatalf("failed to open device: %v", err)
+	}
+	defer f.Close()
+
+	v, err := unix.IoctlGetInt(int(f.Fd()), unix.RNDGETENTCNT)
+	if err != nil {
+		t.Fatalf("failed to perform ioctl: %v", err)
+	}
+
+	t.Logf("%d bits of entropy available", v)
+}
+
 func TestPoll(t *testing.T) {
 	f, cleanup := mktmpfifo(t)
 	defer cleanup()