maps: pagemap, kpagemap, and related cleanups

This patch series introduces /proc/pid/pagemap and /proc/kpagemap,
which allow detailed run-time examination of process memory usage at a
page granularity.

The first several patches whip the page-walking code introduced for
/proc/pid/smaps and clear_refs into a more generic form, the next
couple make those interfaces optional, and the last two introduce the
new interfaces, also optional.