VM working set detection

PG_referenced bitflag
A new page has the referenced bitflag clear
Really "read" pages [not re-referenced pages because of seeks or readahead] are marked PG_referenced
If we try to mark the page PG_referenced and we find that it is inactive but it was just referenced previously (PG_referenced just set) we clear PG_referenced and we move the page into the active_list.
An inactive page will be shrunk regardless of its referenced bitflag (so readahead isn't too much penalized)
An active page with the referenced bitflag set will have a second chance before being refiled into the inactive list.