Preserve the old site-global options by using label names from
All-Projects. Allow custom/project-specific label names with
--label=My-Label=N, which override the old-style options, if present.
When we have project-specific labels, this means the help text will
include e.g. --verified even if that is removed or has different value
in the relevant projects; the command will then fail when the server
actually tries to apply the review score.
Change-Id: I915399556e9e977b36e996071ff0346f76bc0ff5
55b290a92c