re: Issue https://github.com/Unidata/netcdf-c/issues/1365
At some point (4.6.1) we changed the diskless handling of flags
and added a new NC_PERSIST flag. Looks like we did not fix all
occurrences of the old flag set, specifically for 'nccopy -w' command.
Also added some tests (tst_nccopy_w{3,4}.sh) for this situation.