[svn-r18266] Purpose:

bugzilla 1754: h5diff: support comparing through links.
    (original check-in svn revision #18164)
Description:
    fix the hang issue in parallel mode when compare external-link.
    add --no-dangling-links option. 
    add test cases (#450-#459) relate to the new option.
    improve test script to check exit code.
    update --help relate to the new options.
    correct some indentations.

Tested:
    h5committest (jam, amani and linew)

tools/h5diff/testfiles/h5diff_12.txt

@@ -1,2 +1,3 @@
 dataset: </g1/dset1> and </g1/dset2>
 5 differences found
+EXIT CODE: 1