Favor checkID over check_id.

Per golint:

  canaryd.go:55:23: don't use underscores in Go names; func parameter check_id should be checkID
  canaryd.go:64:18: don't use underscores in Go names; func parameter check_id should be checkID
  canaryd.go:68:29: don't use underscores in Go names; func parameter check_id should be checkID
  canaryd.go:75:26: don't use underscores in Go names; func parameter check_id should be checkID
  canaryd.go:107:2: don't use underscores in Go names; var check_id should be checkID