Let the slave PTY keep the master PTY alive.

This ownership model is a bit confusing. There's a retain cycle between
MasterPTY and SlavePTY, but it's broken when the SlavePTY is closed, meaning
that there are no more FileDescriptors referring to it.

Kernel/MasterPTY.cpp

@@ -12,6 +12,7 @@ MasterPTY::MasterPTY(unsigned index)
 
 MasterPTY::~MasterPTY()
 {
+    dbgprintf("~MasterPTY(%u)\n", m_index);
     PTYMultiplexer::the().notify_master_destroyed(Badge<MasterPTY>(), m_index);
 }