The idea here is to get Windows' userspace infrastructure to allocate
whatever space it needs for MapViewOfFileEx() before we release the
locked-down space that we want to map the shared memory block into.

This is a fairly brute-force attempt, and would likely (for example)
fail with large shared memory on 32-bit Windows.  We could perhaps
ameliorate that by mapping only part of the shared memory block in
this way, but for the moment I just want to see if this approach
will fix dory's problem.

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