windows: use pseudo handle constants to implement GetCurrentProcess

There's no point in adding a function call to retrieve a constant, or
worse, a syscall to retrieve a constant. These are fixed and baked so
deep into NT they'll never change. So let's benefit from the obvious
optimization and make these constants. Go easily inlines the function
calls as well. We also take the opportunity to sunset
OpenCurrentProcessToken and restore its original behavior, since users
should be invoking this deliberately with the correct access mask.

Change-Id: I92f7de56c0fcf5b69b59f5a79d2828c7ddf3c8f6
Reviewed-on: https://go-review.googlesource.com/c/sys/+/196800
Run-TryBot: Jason A. Donenfeld <Jason@zx2c4.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Alex Brainman <alex.brainman@gmail.com>

windows/security_windows.go

@@ -647,13 +647,16 @@ func (tml *Tokenmandatorylabel) Size() uint32 {
 // system-related operations on the local computer.
 type Token Handle
 
-// OpenCurrentProcessToken opens the access token
-// associated with current process. It is a real
-// token that needs to be closed, unlike
-// GetCurrentProcessToken.
-func OpenCurrentProcessToken() (token Token, err error) {
-	err = OpenProcessToken(GetCurrentProcess(), TOKEN_QUERY|TOKEN_DUPLICATE, &token)
-	return
+// OpenCurrentProcessToken opens an access token associated with current
+// process with TOKEN_QUERY access. It is a real token that needs to be closed.
+//
+// Deprecated: Explicitly call OpenProcessToken(GetCurrentProcess(), ...)
+// with the desired access instead, or use GetCurrentProcessToken for a
+// TOKEN_QUERY token.
+func OpenCurrentProcessToken() (Token, error) {
+	var token Token
+	err := OpenProcessToken(GetCurrentProcess(), TOKEN_QUERY, &token)
+	return token, err
 }
 
 // GetCurrentProcessToken returns the access token associated with