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/Keyboard.h

@@ -18,6 +18,7 @@ private:
     // ^CharacterDevice
     virtual ssize_t read(byte* buffer, size_t) override;
     virtual ssize_t write(const byte* buffer, size_t) override;
+    virtual bool hasDataAvailableForRead() const override;
 
     CircularQueue<byte, 16> m_queue;
     byte m_modifiers { 0 };