doubleselect.go defines a flag to control the number of iterations,
but never called flag.Parse so it was unusable.

Change-Id: Ib5d0c7119e7f7c9a808dcc02d0d9cc6ba5bbc16e
Reviewed-on: https://go-review.googlesource.com/41299Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>