VFD plugins (#602)

* Implement support for loading of Virtual File Drivers as plugins

Fix plugin caching for VOL connector and VFD plugins

Fix plugin iteration to skip paths that can't be opened

* Enable dynamic loading of VFDs with HDF5_DRIVER environment variable

* Temporarily disable error reporting during H5F_open double file open

* Default to using HDstat in h5_get_file_size for unknown VFDs

* Use macros for some environment variables that HDF5 interprets

* Update "null" and "ctl testing" VFDs

tools/testfiles/pbits/tpbitsIncomplete.ddl

@@ -28,6 +28,12 @@ usage: h5dump [OPTIONS] files
                           HDF5 file specified
      --vol-info           VOL-specific info to pass to the VOL connector used for
                           opening the HDF5 file specified
+     --vfd-value          Value (ID) of the VFL driver to use for opening the
+                          HDF5 file specified
+     --vfd-name           Name of the VFL driver to use for opening the
+                          HDF5 file specified
+     --vfd-info           VFD-specific info to pass to the VFL driver used for
+                          opening the HDF5 file specified
 --------------- Object Options ---------------
      -a P, --attribute=P  Print the specified attribute
                           If an attribute name contains a slash (/), escape the