list enqueue/dequeue, and container_of macros
Makefile: add "+" to calls to make so that the jobs flag works
fork: fork_context() to set up the child for a context switch

Still left to implement:
 - Currently the child and parent share the same physical pages.
   We need to set the pages to read-only and implement copy-on-write
   when there is a page fault.
 - context_switch is currently untested and probably doesn't work.
 - Need to implement a yield() (aka voluntary context switch)
   so we can test context switching.