hdf5_msb

Implement H5VLget_file_type() to return a copy of a datatype with the
location set to be in a file.  Only meant to be used by VOL connectors.
Implement H5VLpeek_connector_id() to support connectors querying their
own IDs.  Fix app_ref with connector IDs in a couple places (external
VOLs registered as default through ENV should be visible to the
application).  Modify vlen and reference interfaces to work with
arbitrary VOL connectors.  Implement file "post open" specific
callback, to enable connectors to update their file structs after a
wrap context has been set.