kernel - SWAP CACHE part 7/many - Add vm_swapcache.c core (write side)

kernel - SWAP CACHE part 7/many - Add vm_swapcache.c core (write side)

* Add vm_swapcache.c which will be responsible for assigning swap to clean
  vnode-backed VM pages and writing the data out.

  Implement a very simple inactive queue scanner and swap-writer for
  testing.

* Track swap space use, split up into the piece used for anonymous
  data and the piece used for clean vnode-backed data.

* Add PG_SWAPPED tracking for newly allocated VM pages via
  swap_pager_page_inserted().

* Conditionalize the swap code's dirtying/undirtying of VM pages.  We
  don't want to mess with the dirty state when working the swap
  cache since it isn't the definitive backing store for the VM page.