Tiny patch: sigmask.diff

Hello

sigmask macro is defined in win32.h like this:

```
#define sigmask(sig) ( 1 << ((sig)-1) )
```

And used in signal.c in this fashion:

```
        for (i = 0; i < PG_SIGNAL_COUNT; i++)
        {
            if (exec_mask & sigmask(i))
            {
```

Thus during first iteration we are doing `<< -1`. I think it's a bug.

Patch attached.

--
Best regards,
Aleksander Alekseev
http://eax.me/