execude: include RuntimeScope field in ExecParameters

Let's decouple execute.c a bit from the Manager object, let's pass the
runtime scope (i.e. the enum that discern invocation for user or system
context) as part of ExecParameters. This makes the scope available in
various functions without having to pass the Manager object in.

src/core/unit.c

@@ -5309,6 +5309,8 @@ int unit_set_exec_params(Unit *u, ExecParameters *p) {
         if (r < 0)
                 return r;
 
+        p->runtime_scope = u->manager->runtime_scope;
+
         p->confirm_spawn = manager_get_confirm_spawn(u->manager);
         p->cgroup_supported = u->manager->cgroup_supported;
         p->prefix = u->manager->prefix;