windows: use correct type for security attributes' descriptor member

The SECURITY_ATTRIBUTES struct always takes a SECURITY_DESCRIPTOR
pointer. Now that we've defined SECURITY_DESCRIPTOR, make
SECURITY_ATTRIBUTES properly specify the type. This eliminates the need
for terrible uintptr(unsafe.Pointer(...)) casts everywhere.

Change-Id: Ibbc85524cfe33589d43f963e10aa19d7f47686f2
Reviewed-on: https://go-review.googlesource.com/c/sys/+/196797
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

@@ -901,6 +901,12 @@ type SECURITY_DESCRIPTOR struct {
 	dacl     *ACL
 }
 
+type SecurityAttributes struct {
+	Length             uint32
+	SecurityDescriptor *SECURITY_DESCRIPTOR
+	InheritHandle      uint32
+}
+
 type SE_OBJECT_TYPE uint32
 
 // Constants for type SE_OBJECT_TYPE