cmd, eth: Add support for --whitelist <blocknum>=<hash>,... flag

* Rejects peers that respond with a different hash for any of the passed in block numbers.
* Meant for emergency situations when the network forks unexpectedly.

cmd/geth/usage.go

@@ -81,6 +81,7 @@ var AppHelpFlagGroups = []flagGroup{
 			utils.LightServFlag,
 			utils.LightPeersFlag,
 			utils.LightKDFFlag,
+			utils.WhitelistFlag,
 		},
 	},
 	{