Clear queue operation flags when migrating a page to another queue.

The page daemon loops may move pages back to the active queue if
references are detected. In this case we must take care to clear
existing queue operation flags. In particular, PGA_REQUEUE_HEAD may be
set, and that flag is only valid if the page belongs to the inactive
queue.

Also fix a bug in the active queue scan where we were updating "old"
instead of "new". This would only have been hit in rare cases where the
page moved out of the active queue after the beginning of the scan.

Reported by: Bob Prohaska, Idwer Vollering
Tested by: Idwer Vollering
Reviewed by: alc, kib
Differential Revision: https://reviews.freebsd.org/D23001