Make private_nets aware of Carrier Grade NAT (RFC 6598)

In legacy/trac#28525 (moved), we make tor_addr_is_internal_() aware of RFC 6598. teor has said that we need a consensus method to do this in private_nets:

It is important
 *  that all authorities agree on that list when creating summaries, so don't
 *  just change this without a proper migration plan and a proposal and stuff.