Modified provenance code to allocate the minimal space
needed for _NCProperties attribute in file. Basically
required using malloc in the provenance code and in ncdump.
Otherwise should cause no externally visible effects.
Also removed the ENABLE_FILEINFO from configure.ac since
the provenance code is no longer optional.
Charlie Zender noted that we forgot to define what happens for
various netcdf API attribute operations, notably nc_inq_att()
and nc_get_att().
So, I added a list of legal and illegal api calls for the provenance
attributes in docs/attribute_conventions.md.
I also added more test cases to ncdump/tst_fileinfo.c to verify
and fixed resultant errors.
This consists of a persistent attribute named
_NCProperties plus two computed attributes
_IsNetcdf4 and _SuperblockVersion.
See the 'Provenance Attributes' section
of docs/attribute_conventions.md for details.
