illumos gate

smf_restarter and svc.startd both make mention of:

single_instance
                          This is an optional property. When set, only one
                          instance of the service is allowed to transition to
                          an online or degraded status at any time.

Except, nothing ever implemented it. And now, due to copy-and-pasting, this is set all over the place in manifests that really do have multiple instances that should run at once. So we can't just implement it (and it was always of highly dubious utility). Let's make clear it's ignored.