[conformal/btcd] 3bfeab: Improve btcctl verifychain usage.

[Dave Collins]

  Branch: refs/heads/master
  Home:   https://github.com/conformal/btcd
  Commit: 3bfeabb47a86ad2d9ce98590f299f8fa04ddb151
      
https://github.com/conformal/btcd/commit/3bfeabb47a86ad2d9ce98590f299f8fa04ddb151
  Author: Dave Collins <da...@conformal.com>
  Date:   2013-12-30 (Mon, 30 Dec 2013)

  Changed paths:
    M util/btcctl/btcctl.go

  Log Message:
  -----------
  Improve btcctl verifychain usage.

Also, add a comment for makeVerifyChain while here.