Towards finishing exchange_registers()

- Added mutex_trylock()
- Implemented most of exchange_registers()
- thread_control() now needs a lock for operations that can modify thread context.
- thread_start() does not initialise scheduler flags, now done in thread_create.

TODO:
- Fork/clone'ed threads should retain their context in tcb, not syscall stack.
- exchange_registers() calls in userspace need cleaning up.

src/generic/scheduler.c

@@ -226,7 +226,7 @@ void sched_notify_resume(struct ktcb *task)
 /* NOTE: Might as well just set need_resched instead of full yield.
  * This would work on irq context as well. */
 /* Same as resume, but also yields. */
-void sched_resume_task(struct ktcb *task)
+int sched_resume_task(struct ktcb *task)
 {
 	sched_notify_resume(task);
 	sched_yield();