improved h2-io-file a bit

h2/lib/win32/h2-sysdef.ads

@@ -0,0 +1,10 @@
+with System;
+
+package H2.Sysdef is
+
+	type PVOID is System.Address;
+	subtype HANDLE is PVOID;
+
+	type DWORD is mod 2 ** 32;
+
+end H2.Sysdef;