Introduce vm_page_pqstate_commit(), which is a generic function that
handles more cases than the specialized vm_page_pqstate_commit_*()
functions.

Implement vm_page_mvqueue() (the backend for vm_page_activate(),
vm_page_deactivate(), etc.), vm_page_unwire() and vm_page_release_toq()
using the new pqstate operations. Use vm_page_release_toq() as the
common back-end for vm_page_unwire() and vm_page_release(). This has
the side effect that vm_page_unwire() will leave the page in the active
queue if it is already there.