Add sk_TYPE_new_reserve() function

This is a combination of sk_new and sk_reserve, to make it more
convenient to allocate a new stack with reserved memory and comaprison
function (if any).

Reviewed-by: Paul Dale <paul.dale@oracle.com>
Reviewed-by: Andy Polyakov <appro@openssl.org>
Reviewed-by: Richard Levitte <levitte@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/4559)