Add a PTY multiplexer (/dev/ptmx) device.

When you open /dev/ptmx, you get a file descriptor pointing to one of the
available MasterPTY's. If none are available, you get an EBUSY.

This makes it possible to open multiple (up to 4) Terminals. :^)

To support this, I also added a CharacterDevice::open() that gets control
when VFS is opening a CharacterDevice. This is useful when we want to return
a custom FileDescriptor like we do here.

Kernel/PTYMultiplexer.h

@@ -0,0 +1,24 @@
+#pragma once
+
+#include <VirtualFileSystem/CharacterDevice.h>
+#include <AK/Lock.h>
+
+class MasterPTY;
+
+class PTYMultiplexer final : public CharacterDevice {
+    AK_MAKE_ETERNAL
+public:
+    PTYMultiplexer();
+    virtual ~PTYMultiplexer() override;
+
+    // ^CharacterDevice
+    virtual RetainPtr<FileDescriptor> open(int& error, int options) override;
+    virtual ssize_t read(Process&, byte*, size_t) override { return 0; }
+    virtual ssize_t write(Process&, const byte*, size_t) override { return 0; }
+    virtual bool can_read(Process&) const override { return true; }
+    virtual bool can_write(Process&) const override { return true; }
+
+private:
+    SpinLock m_lock;
+    Vector<RetainPtr<MasterPTY>> m_freelist;
+};