Add a /dev/pts filesystem and make PTY allocation dynamic.

You can now open as many PTY pairs as you like. Well, it's actually capped
at 8 for now, but it's just a constant and trivial to change.

Unregistering a PTY pair is untested because I didn't want to start
mucking with that in Terminal right now.

Kernel/SyntheticFileSystem.h

@@ -45,6 +45,7 @@ struct SynthFSInodeCustomData {
 
 class SynthFSInode final : public Inode {
     friend class SynthFS;
+    friend class DevPtsFS;
 public:
     virtual ~SynthFSInode() override;