hdf5_ep

Fixed HDFFV-10214:
* Updated comments in the library to accurately reflect the semantics
  of the H5FD_FEAT_POSIX_COMPAT_HANDLE flag.
* Removed the check for the POSIX compatible handle feature flag
  when determining if a VFD supports SWMR.
Partial work for HDFFV-10197:
* Added a new H5FD_FEAT_DEFAULT_VFD_COMPATIBLE feature flag that is
  set when a VFD creates output that is compatible with the default
  VFD. Will be used in the testing but might also be generally useful
  to users.