Incomplete private IP ranges

The [`privateMasks`](https://github.com/sebest/xff/blob/671bd2870b3a94230620f8a68525a3a17e3a4585/xff.go#L11-L18) list is incomplete. Here is a good list to use: 
https://github.com/letsencrypt/boulder/blob/30a516737c9daa4c88c8c47070c25a5e7033cdcf/bdns/dns.go#L31-L145

(Then you can also remove the `ip.IsGlobalUnicast` check.)
