Step 11c of move of engines: Time to make the changes to support

Step 11c of move of engines: Time to make the changes to support
automatic load of dynamic engines.  Change the iterator to try to load
the requested engine dynamically.  The environment variable
OPENSSL_ENGINES can be used to override the internal default directory
where one can expect to find dynamically loadable engines.

Note: The changes in step 11 have all been made by Geoff Thorpe.
Credit where credit is due.