The switch to having an (ENGINE *) handle inside each RSA structure rather

than (RSA_METHOD *) required a couple of functions to change shape. I
didn't really pick the best shape to change RSA_set_method into though. :-)

There's nothing really appropriate to return from RSA_set_method; the
temptation to return an "old handle" fails when you consider that the
caller might ignore the return value and so botch up the reference
counting, this wasn't an issue before because there was no reference
counting.