Support for dumping $D400 registers

```
$ sidplayfp -h
-d<num>      dump SID registers (default: once per frame)

$ sidplayfp -d Cauldron.sid
     0: 973F 945B 60 CFA7  CD7B 37E9 9F 427D  D654 11F8 53 97CA  52
     1: A968 EF9B E2 2D3C  55AB 2301 2B CACC  C552 5E4E 8E B4D7  89
     2: E51C 4666 1C 60FD  FDA5 F36F F4 97B1  0A10 F671 7E C07D  FB
     3: ....
```

