A small pile of Windows compiler-warning fixes.

These include an unused variable left over from the command-line
refactoring; an explicit referencing of the module handle for
sspicli.dll which we really do deliberately load and then don't
(directly) use; a missing pointer-type cast in the Windows handle
socket code; and two 32/64 bit integer size mismatches in the types of
functions I was importing from system API DLLs.

The last of those are a bit worrying, and suggest to me that after
going to all that trouble to add type-checking of those runtime
imports in commit 49fb598b0, I might have only checked the resulting
compiler output in a 32-bit build and not a 64-bit one. Oops!

windows/winhsock.c

@@ -194,7 +194,7 @@ static void handle_socket_unfreeze(void *psv)
     bufchain_consume(&ps->inputdata, len);
     ps->defer_close = FALSE;
     if (ps->deferred_close) {
-        sk_handle_close(ps);
+        sk_handle_close((Socket)ps);
         return;
     }