Refactor ipv[46]_policy accessors in tor_netdir::Relay to account for BadExit

Right now the IP policy accessors on Relay do not check the BadExit flag. That's error-prone for many uses: it would be better to return "reject *" if BadExit is set. We should have that be the behavior for ipv4_policy() and ipv6_policy(). We should also add secondary accessor that return the declared policy even if the BadExit flag is set.