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/init.cpp

@@ -142,6 +142,7 @@ void init()
     gdt_init();
     idt_init();
 
+    PTYMultiplexer::initialize_statics();
     VFS::initialize_globals();
     vfs = new VFS;