Add basic PTY support.

For now, there are four hard-coded PTYs: /dev/pt{m,s}[0123]
Use this in the Terminal to open a pty pair and spawn a shell.

Kernel/SlavePTY.h

@@ -0,0 +1,24 @@
+#pragma once
+
+#include "TTY.h"
+
+class MasterPTY;
+
+class SlavePTY final : public TTY {
+public:
+    explicit SlavePTY(unsigned index);
+    virtual ~SlavePTY() override;
+    void set_master(MasterPTY& master) { m_master = &master; }
+
+    virtual String tty_name() const override;
+
+    void on_master_write(const byte*, size_t);
+
+protected:
+    virtual void on_tty_write(const byte*, size_t) override;
+
+private:
+    unsigned m_index;
+    MasterPTY* m_master { nullptr };
+};
+