Cleaner disposal of ephemeral engine ids and names

Engine names and ids are typically static strings.  If an application
actually dynamically allocated these, the application owns the
storage, and should dispose of it via the original handle, rather
than the "const char *" returned by the engine.

In any case, this resolves the test code issue without resort to
"unconst" macros/casts.

Reviewed-by: Richard Levitte <levitte@openssl.org>