On google fs (drive, google photos, and google cloud storage), if
headless is selected, do not open browser.
This also supplies a new option "auth-no-open-browser" for authorize
if the user does not want it.
This should fix#3323.
The purpose of this is to make it easier to maintain and eventually to
allow the rclone backends to be re-used in other projects without
having to use the rclone configuration system.
The new code layout is documented in CONTRIBUTING.