core: enforce that scope units can be started only once

Scope units are populated from PIDs specified by the bus client. We do
that when a scope is started. We really shouldn't allow scopes to be
started multiple times, as the PIDs then might be heavily out of date.
Moreover, clients should have the guarantee that any scope they allocate
has a clear runtime cycle which is not repetitive.

src/core/scope.c

@@ -587,6 +587,7 @@ const UnitVTable scope_vtable = {
 
         .can_transient = true,
         .can_delegate = true,
+        .once_only = true,
 
         .init = scope_init,
         .load = scope_load,