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/MasterPTY.cpp

@@ -1,15 +1,18 @@
 #include "MasterPTY.h"
 #include "SlavePTY.h"
+#include "PTYMultiplexer.h"
+#include <LibC/errno_numbers.h>
 
 MasterPTY::MasterPTY(unsigned index)
     : CharacterDevice(10, index)
-    , m_slave(*new SlavePTY(*this, index))
+    , m_slave(adopt(*new SlavePTY(*this, index)))
     , m_index(index)
 {
 }
 
 MasterPTY::~MasterPTY()
 {
+    PTYMultiplexer::the().notify_master_destroyed(Badge<MasterPTY>(), m_index);
 }
 
 String MasterPTY::pts_name() const
@@ -19,17 +22,23 @@ String MasterPTY::pts_name() const
 
 ssize_t MasterPTY::read(Process&, byte* buffer, size_t size)
 {
+    if (!m_slave && m_buffer.is_empty())
+        return 0;
     return m_buffer.read(buffer, size);
 }
 
 ssize_t MasterPTY::write(Process&, const byte* buffer, size_t size)
 {
-    m_slave.on_master_write(buffer, size);
+    if (!m_slave)
+        return -EIO;
+    m_slave->on_master_write(buffer, size);
     return size;
 }
 
 bool MasterPTY::can_read(Process&) const
 {
+    if (!m_slave)
+        return true;
     return !m_buffer.is_empty();
 }
 
@@ -38,6 +47,15 @@ bool MasterPTY::can_write(Process&) const
     return true;
 }
 
+void MasterPTY::notify_slave_closed(Badge<SlavePTY>)
+{
+    dbgprintf("MasterPTY(%u): slave closed, my retains: %u, slave retains: %u\n", m_index, retain_count(), m_slave->retain_count());
+    // +1 retain for my MasterPTY::m_slave
+    // +1 retain for FileDescriptor::m_device
+    if (m_slave->retain_count() == 2)
+        m_slave = nullptr;
+}
+
 void MasterPTY::on_slave_write(const byte* data, size_t size)
 {
     m_buffer.write(data, size);