(WIP) cmd: Reimplement console buffer system as a linked list

(WIP) cmd: Reimplement console buffer system as a linked list

This has several advantages, namely, multiple command interpreters
may share the same console buffer on the same thread, and each
command can be executed in order, eliminating any potential bugs
related to that.

Things left to do:

* Merge deferred commands into the same system and reenable
  the defer command.
* Use a cyclic doubly linked list instead of the garbage I
  wrote.