hdf5_msb

Consolidate VFD SWMR variable declarations in a new header file,
H5FDvfd_swmr_private.h.

Perform tick processing in FUNC_ENTER_API_NOCLEAR, where it was missing.

Track the number of times the HDF5 library has been entered/exited through its
public API.  Only perform tick processing on the first entry and last exit.
This stops us from performing tick processing in API calls invoked by
application callbacks.  Performing tick processing in nested API calls led to
crashes.

Note well: FUNC_LEAVE_API now performs tick processing even on an error exit!
Previously, it did not.  I'm not sure if the change is ok.