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/NullDevice.cpp

@@ -11,6 +11,11 @@ NullDevice::~NullDevice()
 {
 }
 
+bool NullDevice::hasDataAvailableForRead() const
+{
+    return true;
+}
+
 Unix::ssize_t NullDevice::read(byte*, Unix::size_t)
 {
     kprintf("NullDevice: read from null\n");