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.