Callback revision.

Callback revision.

Use "parse" and "add" for function and callback names instead of
"first" and "second".

Change arguments to callback so the extension type is unsigned int
and the buffer length is size_t. Note: this *will* break existing code.
Reviewed-by: Emilia Käsper <emilia@openssl.org>
(cherry picked from commit de2a9e38f39eacc2e052d694f5b5fa5b7e734abc)