Add some temporary code to record stack usage at server process exit.

This patch is meant to gather information from the buildfarm members, and
will be reverted in a day or so.  The idea is to try to find out the
high-water stack consumption while running the regression tests,
particularly on IA64 which is suspected to use much more stack than other
architectures.  On machines with pmap, we can use that; but the IA64 farm
members are running HPUX, so also include some bespoke code for HPUX.
(I've tested the latter on HPUX 10/HPPA; not entirely sure it will work
on HPUX 11/IA64, but we'll soon find out.)

Discussion: <CAM-w4HMwwcwaVvYcAH0_FGtG5GeXdYVRfvG81pXnSJWHnCfosQ@mail.gmail.com>