Implement a basic way for read() to block.

FileHandle gets a hasDataAvailableForRead() getter.
If this returns true in sys$read(), the task will block(BlockedRead) + yield.
The fd blocked on is stored in Task::m_fdBlockedOnRead.
The scheduler then looks at the state of that fd during the unblock phase.

This makes "sh" restful. :^)

There's still some problem with the kernel not surviving the colonel task
getting scheduled. I need to figure that out and fix it.

Kernel/init.cpp

@@ -165,7 +165,7 @@ static void init_stage2()
 #endif
 
     for (;;) {
-        sleep(3600 * TICKS_PER_SECOND);
+        //sleep(3600 * TICKS_PER_SECOND);
         asm("hlt");
     }
 }