ENGINE's init() and finish() handler functions are used when the ENGINE is

being enabled or disabled (respectively) for operation. Additionally, each
ENGINE has a constructor function where it can do more 'structural' level
intialisations such as loading error strings, creating "ex_data" indices,
etc. This change introduces a handler function that gives an ENGINE a
corresponding opportunity to cleanup when the ENGINE is being destroyed. It
also adds the "get/set" API functions that control this "destroy" handler
function in an ENGINE.

crypto/engine/engine_int.h

@@ -112,6 +112,7 @@ struct engine_st
 
 	BN_MOD_EXP bn_mod_exp;
 	BN_MOD_EXP_CRT bn_mod_exp_crt;
+	ENGINE_GEN_INT_FUNC_PTR	destroy;
 	ENGINE_GEN_INT_FUNC_PTR init;
 	ENGINE_GEN_INT_FUNC_PTR finish;
 	ENGINE_CTRL_FUNC_PTR ctrl;