hdf5_ep

[svn-r23702] Issue 8380 - H5Zunregister caused seg fault.  This is a porting of code from the trunk.  It fixes the problem by first iterating 
through all opened datasets and groups to see if any of them uses the filter.  If it finds one, the function fails with a message.  
Then the function flushes all opened files.

Tested with h5committest.