This commit selects the best HDF5 format compatibility options when re-opening an existing netCDF-4 file for writing, such as appending, or adding new groups or variables.

The general objective is to make netCDF-4 files that can be read and written by all previous library  versions.  Optimal HDF5 v1.8 compatibility is selected whenever possible.  Otherwise this falls back to the adequate v1.6 compatibility.

Format compatibility is a transient property of the HDF5 library, rather than baked in at file creation time.  Therefore, compatibility options must be re-selected every time a netCDF-4 file is re-opened for writing.

This builds on the previous update for initial file creation, PR Unidata#1931, by @brtnfld, released in netcdf-c version 4.8.1.

In particular, this commit moves compatibility controls into a single central location, a new common function that is shared by both create and open functions.

For more details, see issue Unidata#951, also documentation at the top of libhdf5/hdf5set_format_compatibility.c.

This commit also makes several corrections and cleanups to previous comments about the use of related property lists.