This resolves two issues:
* the checks in rend_add_services were only being performed when adding
  the service, and not when the service was validated,
  (this meant that duplicate checks were not being performed, and some SETCONF
  commands appeared to succeed when they actually failed), and
* if one service failed while services were being added, then the service
  list would be left in an inconsistent state (tor dies when this happens,
  but the code is cleaner now).

Fixes #20860.