A) Fix for HDFFV-1238:
--Add 3 new options to allow users in setting threshold for small groups/datasets/attributes (tools/h5stat/h5stat.c)
--Generate new test file for testing the new options (tools/h5stat/h5stat_gentest.c)
--Add tests for the new options (tools/h5stat/testh5stat.sh.in)
--Update expected output files (tools/h5stat/testfiles/*.ddl)
B) Fix a bug in determining maximum dimension size for 1-D dataset in dataset_stats() (tools/h5stat/h5stat.c)
C) Fix a bug when using opt_arg that is NULL for the new options.