Add sk_new_reserve support

This is a specific 1.1.1 change; do not squash if the chacha
prioritization code is to be backported

Reviewed-by: Paul Dale <paul.dale@oracle.com>
Reviewed-by: Tim Hudson <tjh@openssl.org>
Reviewed-by: Andy Polyakov <appro@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/4436)