Fix Denial of Service vulnerability in named(8). [13:07]

Security:	CVE-2013-4854
Security:	FreeBSD-SA-13:07.bind
Approved by:	so
Improve page LRU quality and simplify the logic.

 - Don't short-circuit aging tests for unmapped objects.  This biases
   against unmapped file pages and transient mappings.
 - Always honor PGA_REFERENCED.  We can now use this after soft busying
   to lazily restart the LRU.
 - Don't transition directly from active to cached bypassing the inactive
   queue.  This frees recently used data much too early.
 - Rename actcount to act_delta to be more consistent with use and meaning.

Reviewed by:	kib, alc
Sponsored by:	EMC / Isilon Storage Division