actool/manifest.go: support patching app.SupplementaryGroups
actool/manifest.go: support patching app.SupplementaryGroups

Adds --supplementary-groups flag to actool which takes a comma-separated
list of gids.

apcera/appc-spec,jonboulle/spec,apcera/appc-spec,appc/spec,jonboulle/spec,appc/spec,squeed/spec,3ofcoins/appc-spec,dgonyeo/spec,apcera/appc-spec,3ofcoins/appc-spec,dgonyeo/spec,appc/spec,3ofcoins/appc-spec,jonboulle/spec,squeed/spec,squeed/spec,dgonyeo/spec