* 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
