kernel - SWAP CACHE part 20/many - add 'cache' and 'noscache' chflags.

kernel - SWAP CACHE part 20/many - add 'cache' and 'noscache' chflags.

* Allow directory hierarchies to be selected for data caching when
  using vm.swapcache.data_enable.

* Add the vm.swapcache.use_chflags sysctl which defaults to ON and
  enables use of the new chflags flags to determine what directory
  trees the swapcache will cache data from.

* Add chflags cache and noscache.  The flags are tracked recursively
  by the namecache and do *NOT* have to be set recursively in the
  directory tree.  Setting a flag in a top-level directory is sufficient
  to cover the entire subtree.

  chflags cache -	Any regular file in the subtree will be cached
			by swapcache.

  chflags noscache -	Disables any swapcacheing of data in the subtree,
			overrides any use of chflags cache in the subtree.

  NOTE: Only applies to file data.  The caching of file meta-data by
  swapcache is controlled globally by vm.swapcache.meta_enable and
  ignores chflags flags..

* Adjust the manual pages for swapcache and chflags.

* NOTE! The default has been changed to require the use of chflags, data
  caching will not occur unless you either turn off the
  vm.swapcache.use_chflags sysctl (which enables data caching globally)
  or do something like 'chflags cache /'.  Of course vm.swapcache.read_enable
  must also be turned on for swapcache to cache file data.

* NOTE! World must be rebuilt for libc, chflags, and ls to understand the
  new flags.