Deallocate PTY's when they close.

This required a fair bit of plumbing. The CharacterDevice::close() virtual
will now be closed by ~FileDescriptor(), allowing device implementations to
do custom cleanup at that point.

One big problem remains: if the master PTY is closed before the slave PTY,
we go into crashy land.

Kernel/PTYMultiplexer.h

@@ -1,6 +1,7 @@
 #pragma once
 
 #include <Kernel/CharacterDevice.h>
+#include <AK/Badge.h>
 #include <AK/Lock.h>
 
 class MasterPTY;
@@ -11,6 +12,9 @@ public:
     PTYMultiplexer();
     virtual ~PTYMultiplexer() override;
 
+    static PTYMultiplexer& the();
+    static void initialize_statics();
+
     // ^CharacterDevice
     virtual RetainPtr<FileDescriptor> open(int& error, int options) override;
     virtual ssize_t read(Process&, byte*, size_t) override { return 0; }
@@ -18,6 +22,8 @@ public:
     virtual bool can_read(Process&) const override { return true; }
     virtual bool can_write(Process&) const override { return true; }
 
+    void notify_master_destroyed(Badge<MasterPTY>, unsigned index);
+
 private:
     // ^CharacterDevice
     virtual const char* class_name() const override { return "PTYMultiplexer"; }