Merge all of my changes from merge-back-to-feature-vfd_swmr-attempt-1,

including the merge of `hdffv/hdf5/develop`, back to the branch that Vailin and
I share.

Now I need to put this branch on a fork with a less confusing name than
vchoi_fork!

tools/test/h5diff/testfiles/h5diff_udfail.err

@@ -0,0 +1,12 @@
+
+file1     file2
+---------------------------------------
+    x      x    /              
+    x      x    /dynlibud      
+
+group  : </> and </>
+0 differences found
+dataset: </dynlibud> and </dynlibud>
+0 differences found
+Warning: dataset </dynlibud> cannot be read, user defined filter is not available
+EXIT CODE: 2