Move from struct proc to process the reference-count-holding pointers

Move from struct proc to process the reference-count-holding pointers
to the process's vmspace and filedescs.  struct proc continues to
keep copies of the pointers, copying them on fork, clearing them
on exit, and (for vmspace) refreshing on exec.
Also, make uvm_swapout_threads() thread aware, eliminating p_swtime
in kernel.

particular testing by ajacoutot@ and sebastia@