This patch logically continues previous change `bigfile/virtmem:
Introduce "mmap overlay" mode` and exposes mmap-overlay functionality to
Python: if PyBigFile backend provides .blkmmapper PyCapsule the
mmap-related methods will be extracted from it and passed on through to
virtmem - see _bigfile.h for details.

ZBigFile will use this to hook into using WCFS.