Userland 1:1 threading changes step 1/4+:

Userland 1:1 threading changes step 1/4+:

o Move thread-local members from struct proc into new struct lwp.

o Add a LIST_HEAD(lwp) p_lwps to struct proc.  This links a proc
  with its lwps.

o Add a td_lwp member to struct thread which links a thread to its lwp,
  if it exists.  This won't replace td_proc completely to save indirections.

o For now embed one struct lwp into struct proc and set up preprocessor
  linkage so that semantics don't change for the rest of the kernel.
  Once all consumers are converted to take a struct lwp instead of a struct
  proc, this will go away.

Reviewed-by: dillon, davidxu