Nonblocking mail_receive, fix proc_kill scheduling issues

kernel/proc/proc.h

@@ -54,7 +54,7 @@ struct proc {
   struct proc_env env;
 };
 
-void proc_sched (void);
+void proc_sched (bool user);
 
 void proc_kill (struct proc* proc, struct reschedule_ctx* rctx);