This morning's results from buildfarm member dory make it pretty
clear that something is getting mapped into the just-freed space,
but not what that something is.  Replace my minimalistic probes
with a full dump of the process address space and module space,
based on Noah's work at
<20170403065106.GA2624300%40tornado.leadboat.com>

This is all (probably) to get reverted once we have fixed the
problem, but for now we need information.

Discussion: https://postgr.es/m/25495.1524517820@sss.pgh.pa.us