services autorun: new facility, off by default, to auto-run services/autorun/*.cf

(cherry picked from commit 36efa013908fb6efd9d6c6da7e3ebe5a7ddca910)
promises.cf: guard from 3.4 evaluation of the autorun bundle
Merge pull request #225 from tzz/services_autorun_3.6.x

Services autorun 3.6.x