Kernel: Reading from a slave PTY should give EOF if master PTY is closed.

Kernel/MasterPTY.cpp

@@ -69,3 +69,12 @@ bool MasterPTY::can_write_from_slave() const
 {
     return m_buffer.bytes_in_write_buffer() < 4096;
 }
+
+void MasterPTY::close()
+{
+    if (retain_count() == 2) {
+        // After the closing FileDescriptor dies, slave is the only thing keeping me alive.
+        // From this point, let's consider ourselves closed.
+        m_closed = true;
+    }
+}