unix: add IoctlSetPointerInt

IoctlSetPointerInt is necessary for interacting with the PPP kernel
driver, which wants it passed as a pointer to int, rather than the
more conventional int cast as a pointer.

We can technically do this already with
IoctlSetInt(int(uintptr(unsafe.Pointer(&foo)))), but that's just
masking the operation we're trying to execute in the first place,
and relying on the internals of IoctlSetInt to do the right
inverse transformation.

Change-Id: I1e6a1dd6190c11f06c6f5393cd2cae453939878a
GitHub-Last-Rev: a88f56073d04b02d9c076b112adaf99169a49513
GitHub-Pull-Request: golang/sys#25
Reviewed-on: https://go-review.googlesource.com/c/150321
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

unix/syscall_linux.go

@@ -57,6 +57,15 @@ func Fchmodat(dirfd int, path string, mode uint32, flags int) (err error) {
 // ioctl itself should not be exposed directly, but additional get/set
 // functions for specific types are permissible.
 
+// IoctlSetPointerInt performs an ioctl operation which sets an
+// integer value on fd, using the specified request number. The ioctl
+// argument is called with a pointer to the integer value, rather than
+// passing the integer value directly.
+func IoctlSetPointerInt(fd int, req uint, value int) error {
+	v := int32(value)
+	return ioctl(fd, req, uintptr(unsafe.Pointer(&v)))
+}
+
 // IoctlSetInt performs an ioctl operation which sets an integer value
 // on fd, using the specified request number.
 func IoctlSetInt(fd int, req uint, value int) error {