switch-to-configuration: Add new option X-OnlyManualStart

This is to facilitate units that should _only_ be manually started and
not activated when a configuration is switched to.

More specifically this is to be used by the new Nixops deploy-*
targets created in https://github.com/NixOS/nixops/pull/1245 that are
triggered by Nixops before/after switch-to-configuration is called.