windows: revert breaking API changes from CL 196798, add new accessor funcs

CL 196798 changed API and broke a number of projects.

Revert the signature changes but add new convenience functions instead.

Change-Id: I49e389204f4756ec054ba8fd7555e235ef6370e8
Reviewed-on: https://go-review.googlesource.com/c/sys/+/197597
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>

windows/syscall_windows.go

@@ -309,16 +309,32 @@ func NewCallbackCDecl(fn interface{}) uintptr {
 
 // GetCurrentProcess returns the handle for the current process.
 // It is a pseudo handle that does not need to be closed.
-func GetCurrentProcess() Handle {
-	return Handle(^uintptr(1 - 1))
+// The returned error is always nil.
+//
+// Deprecated: use CurrentProcess for the same Handle without the nil
+// error.
+func GetCurrentProcess() (Handle, error) {
+	return CurrentProcess(), nil
 }
 
+// CurrentProcess returns the handle for the current process.
+// It is a pseudo handle that does not need to be closed.
+func CurrentProcess() Handle { return Handle(^uintptr(1 - 1)) }
+
 // GetCurrentThread returns the handle for the current thread.
 // It is a pseudo handle that does not need to be closed.
-func GetCurrentThread() Handle {
-	return Handle(^uintptr(2 - 1))
+// The returned error is always nil.
+//
+// Deprecated: use CurrentThread for the same Handle without the nil
+// error.
+func GetCurrentThread() (Handle, error) {
+	return CurrentThread(), nil
 }
 
+// CurrentThread returns the handle for the current thread.
+// It is a pseudo handle that does not need to be closed.
+func CurrentThread() Handle { return Handle(^uintptr(2 - 1)) }
+
 // GetProcAddressByOrdinal retrieves the address of the exported
 // function from module by ordinal.
 func GetProcAddressByOrdinal(module Handle, ordinal uintptr) (proc uintptr, err error) {