Implement proper wait/waitpid.

This is a pretty big commit, so let's run through it in parts:

- All of the userspace changes are to switch away from syscall_wait
  Mostly, this is to waitpid; some things were tweaked to do things
  "properly" instead of waiting for particular processes. Init has
  been fixed to do a proper spin wait.
- syscall_wait is gone - as are its uses. newlib bindings have been
  using just waitpid for a while now.
- waitpid now performs like a Unix waitpid
- process reaping is no longer a "do this on next change thing":
  it happens when a process is waited on, like it should
  (That means we can have real zombies: terminated processes that
  have not yet been waited on)
- Reparenting of children to init has been implemented, so you
  can fork-daemonize!

Overall, this is pretty big... So I hope it doesn't break everything.