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/RandomDevice.cpp

@@ -28,6 +28,11 @@ static void mysrand(unsigned seed)
 }
 #endif
 
+bool RandomDevice::hasDataAvailableForRead() const
+{
+    return true;
+}
+
 Unix::ssize_t RandomDevice::read(byte* buffer, Unix::size_t bufferSize)
 {
     const int range = 'z' - 'a';