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/SlavePTY.cpp

@@ -15,6 +15,7 @@ SlavePTY::SlavePTY(MasterPTY& master, unsigned index)
 SlavePTY::~SlavePTY()
 {
     DevPtsFS::the().unregister_slave_pty(*this);
+    VFS::the().unregister_character_device(*this);
 }
 
 String SlavePTY::tty_name() const
@@ -37,3 +38,8 @@ bool SlavePTY::can_write(Process&) const
 {
     return m_master.can_write_from_slave();
 }
+
+void SlavePTY::close()
+{
+    m_master.notify_slave_closed(Badge<SlavePTY>());
+}