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Provide integration with virtmem, so that WCFS Mapping can be associated
and managed under virtmem VMA. In other words provide support so that WCFS can
be used as ZBigFile backend in "mmap overlay" mode (see fae045cc "bigfile/virtmem:
Introduce "mmap overlay" mode" for description of mmap-overlay mode).

We'll need this functionality for ZBigFile + WCFS client integration.

Virtmem integration will be tested via running whole wendelin.core functional
testsuite in wcfs-mode after the next patch.

Quoting added description:

---- 8< ----

Integration with wendelin.core virtmem layer
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

This client package can be used standalone, but additionally provides
integration with wendelin.core userspace virtual memory manager: when a
Mapping is created, it can be associated as serving base layer for a
particular virtmem VMA via FileH.mmap(vma=...). In that case, since virtmem
itself adds another layer of dirty pages over read-only b...