windows,windows\svc,windows\svc\mgr: use unsafe.Slice instead of unsafeheader.Slice

unsafe.Slice is available since Go 1.17, which is already the minimum
version supported by this package.

This change removes the dependency on the internal unsafeheader package,
which can be removed from the module.

Change-Id: I6c34cb152f2336ea04c5f9c7e88797ed8914f9cc
Reviewed-on: https://go-review.googlesource.com/c/sys/+/526635
Run-TryBot: Quim Muntal <quimmuntal@gmail.com>
Reviewed-by: Bryan Mills <bcmills@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Cherry Mui <cherryyz@google.com>

windows/security_windows.go

@@ -7,8 +7,6 @@ package windows
 import (
 	"syscall"
 	"unsafe"
-
-	"golang.org/x/sys/internal/unsafeheader"
 )
 
 const (
@@ -1341,21 +1339,14 @@ func (selfRelativeSD *SECURITY_DESCRIPTOR) copySelfRelativeSecurityDescriptor()
 		sdLen = min
 	}
 
-	var src []byte
-	h := (*unsafeheader.Slice)(unsafe.Pointer(&src))
-	h.Data = unsafe.Pointer(selfRelativeSD)
-	h.Len = sdLen
-	h.Cap = sdLen
-
+	src := unsafe.Slice((*byte)(unsafe.Pointer(selfRelativeSD)), sdLen)
+	// SECURITY_DESCRIPTOR has pointers in it, which means checkptr expects for it to
+	// be aligned properly. When we're copying a Windows-allocated struct to a
+	// Go-allocated one, make sure that the Go allocation is aligned to the
+	// pointer size.
 	const psize = int(unsafe.Sizeof(uintptr(0)))
-
-	var dst []byte
-	h = (*unsafeheader.Slice)(unsafe.Pointer(&dst))
 	alloc := make([]uintptr, (sdLen+psize-1)/psize)
-	h.Data = (*unsafeheader.Slice)(unsafe.Pointer(&alloc)).Data
-	h.Len = sdLen
-	h.Cap = sdLen
-
+	dst := unsafe.Slice((*byte)(unsafe.Pointer(&alloc[0])), sdLen)
 	copy(dst, src)
 	return (*SECURITY_DESCRIPTOR)(unsafe.Pointer(&dst[0]))
 }