cmd/geth: add db-command to inspect freezer index (ref #22111)

[holiman]

This PR makes it easier to inspect the freezer index, which could be useful to investigate things like #22111

geth --yolov3 db freezer-index diffs  214043 214056
INFO [04-08|13:52:42.921] Maximum peer count                       ETH=50 LES=0 total=50
INFO [04-08|13:52:42.921] Smartcard socket not found, disabling    err="stat /run/pcscd/pcscd.comm: no such file or directory"
INFO [04-08|13:52:42.922] Set global gas cap                       cap=25000000
INFO [04-08|13:52:42.922] Opening freezer                          location=/home/user/.ethereum/yolo-v3/geth/chaindata/ancient name=diffs
| number | fileno | offset |
|--------|--------|--------|
|  214043   |  000   |  823212   | 
|  214044   |  000   |  823216   | 
|  214045   |  000   |  823220   | 
|  214046   |  000   |  823224   | 
|  214047   |  000   |  823228   | 
|  214048   |  000   |  823232   | 
|  214049   |  000   |  823236   | 
|  214050   |  000   |  823240   | 
|  214051   |  000   |  823244   | 
|  214052   |  000   |  823248   | 
|  214053   |  000   |  823252   | 
|  214054   |  000   |  823256   | 
|  214055   |  000   |  823260   | 
|  214056   |  000   |  823264   | 
|--------------------------|