Added skeletal version of H5FDsubfiling.c and H5FDsubfiling.h.inactive/develop_subfiling

These are based on H5FDsec2, and will have to be re-worked heavily
for sub-filing.

Also added minimal test (based on existing sec2 tests) in test/vfd.c
Since sub-filing is mostly a parallel feature, most testing will have
to be moved to testpar.  That said, a serial version of sub-filing will
be neeced eventually, so it makes sense to retain at least a stub in
the serial vfd tests.

Tested serial and parallel on charis and jelly.  Encountered errors in
h5diff tests in the parallel builds, but am ignoring them as they also
appear in the version prior to this checkin.

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