PTY: Disallow infinite writing to slaves.

This way we don't buffer ungodly amounts of output in the kernel when doing
e.g "cat /dev/random" on a PTY.

Kernel/MasterPTY.h

@@ -20,6 +20,7 @@ public:
     unsigned index() const { return m_index; }
     String pts_name() const;
     void on_slave_write(const byte*, size_t);
+    bool can_write_from_slave() const;
 
 private:
     // ^CharacterDevice