Kernel: When a lock is busy, donate remaining process ticks to lock holder.

Since we know who's holding the lock, and we're gonna have to yield anyway,
we can just ask the scheduler to donate any remaining ticks to that process.

Kernel/Scheduler.h

@@ -17,6 +17,7 @@ public:
     static void pick_next_and_switch_now();
     static void switch_now();
     static bool yield();
+    static bool donate_to(Process*, const char* reason);
     static bool context_switch(Process&);
     static void prepare_to_modify_tss(Process&);
     static Process* colonel();