Some more tweaks to ENGINE code.

* Constify the get/set functions, and add some that functions were missing.

* Add a new 'ENGINE_cpy()' function that will produce a new ENGINE based
  copied from an original (except for the references, ie. the new copy will
  be like an ENGINE returned from 'ENGINE_new()' - a structural reference).

* Removed the "null parameter" checking in the get/set functions - it is
  legitimate to set NULL values as a way of *changing* an ENGINE (ie.
  removing a handler that previously existed). Also, passing a NULL pointer
  for an ENGINE is obviously wrong for these functions, so don't bother
  checking for it. The result is a number of error codes and strings could
  be removed.