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.cpp

@@ -86,6 +86,11 @@ Keyboard::~Keyboard()
     ASSERT_NOT_REACHED();
 }
 
+bool Keyboard::hasDataAvailableForRead() const
+{
+    return !m_queue.isEmpty();
+}
+
 ssize_t Keyboard::read(byte* buffer, size_t size)
 {
     ssize_t nread = 0;