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with the intention to later use WCFS through it.

Before this patch virtmem had only one mode: a BigFile backend was
providing loadblk and storeblk methods, and on every block access
loadblk was called to load block data into allocated RAM page.

However with WCFS virtmem won't be needed to do anything to load data -
because loading from head/bigfile/f mmaped through OS will be handled by
OS directly. Thus for wcfs, that leaves virtmem only to handle dirtying
and writeout.

-> Introduce "mmap overlay" mode into virtmem to handle WCFS-like
BigFile backends - that can provide read-only base layer suitable for
mmapping.

This patch is organized as follows:

- fileh_open is added flags argument to indicate which mode to use for
  opened fileh. BigFileH is added .mmap_overlay bitfield correspondingly.
  (virtmem.h)

- struct bigfile_ops is extended with 3 optional methods that a BigFile
  backend might provide to support mmap-overlay mode:

  * mmap_setup_read,
  ...