Fix/enhance THP integration.

Detect whether chunks start off as THP-capable by default (according to
the state of /sys/kernel/mm/transparent_hugepage/enabled), and use this
as the basis for whether to call pages_nohuge() once per chunk during
first purge of any of the chunk's page runs.

Add the --disable-thp configure option, as well as the the opt.thp
mallctl.

This resolves #541.