PTY: Disallow infinite writing to slaves.

This way we don't buffer ungodly amounts of output in the kernel when doing
e.g "cat /dev/random" on a PTY.

Kernel/MasterPTY.cpp

@@ -45,3 +45,8 @@ void MasterPTY::on_slave_write(const byte* data, size_t size)
 {
     m_buffer.write(data, size);
 }
+
+bool MasterPTY::can_write_from_slave() const
+{
+    return m_buffer.bytes_in_write_buffer() < 4096;
+}

Kernel/MasterPTY.h

@@ -20,6 +20,7 @@ public:
     unsigned index() const { return m_index; }
     String pts_name() const;
     void on_slave_write(const byte*, size_t);
+    bool can_write_from_slave() const;
 
 private:
     // ^CharacterDevice

Kernel/SlavePTY.cpp

@@ -31,7 +31,7 @@ void SlavePTY::on_tty_write(const byte* data, size_t size)
     m_master.on_slave_write(data, size);
 }
 
-bool SlavePTY::can_write(Process& process) const
+bool SlavePTY::can_write(Process&) const
 {
-    return m_master.can_write(process);
+    return m_master.can_write_from_slave();
 }