Add basic PTY support.

For now, there are four hard-coded PTYs: /dev/pt{m,s}[0123]
Use this in the Terminal to open a pty pair and spawn a shell.

Kernel/MasterPTY.cpp

@@ -0,0 +1,42 @@
+#include "MasterPTY.h"
+#include "SlavePTY.h"
+
+MasterPTY::MasterPTY(unsigned index)
+    : CharacterDevice(10, index)
+    , m_index(index)
+{
+
+}
+
+MasterPTY::~MasterPTY()
+{
+}
+
+String MasterPTY::pts_name() const
+{
+    dbgprintf("MasterPTY::pts_name requested for index %u!\n", m_index);
+    char buffer[32];
+    ksprintf(buffer, "/dev/pts%u", m_index);
+    return buffer;
+}
+
+ssize_t MasterPTY::read(byte* buffer, size_t size)
+{
+    return m_buffer.read(buffer, size);
+}
+
+ssize_t MasterPTY::write(const byte* buffer, size_t size)
+{
+    m_slave->on_master_write(buffer, size);
+    return size;
+}
+
+bool MasterPTY::has_data_available_for_reading(Process&) const
+{
+    return !m_buffer.is_empty();
+}
+
+void MasterPTY::on_slave_write(const byte* data, size_t size)
+{
+    m_buffer.write(data, size);
+}