* 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.
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