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.

VirtualFileSystem/FileHandle.cpp

@@ -107,6 +107,13 @@ Unix::ssize_t FileHandle::read(byte* buffer, Unix::size_t count)
     return nread;
 }
 
+bool FileHandle::hasDataAvailableForRead()
+{
+    if (m_vnode->isCharacterDevice())
+        return m_vnode->characterDevice()->hasDataAvailableForRead();
+    return true;
+}
+
 ByteBuffer FileHandle::readEntireFile()
 {
     Locker locker(VirtualFileSystem::lock());