Replace the number 4 in pmap_ts_referenced() by PMAP_TS_REFERENCED_MAX,
like we've done elsewhere, e.g., amd64.

As an optimization to the machine-independent layer, change the machine-
dependent pmap_ts_referenced() so that it updates the page's dirty field
if a modified bit is found while counting reference bits. This
opportunistic update can be performed at low cost and can eliminate the
need for some future calls to pmap_is_modified() by the machine-
independent layer.

MFC after: 3 weeks