Bug 5241: Block to-localhost, to-link-local requests by default (#1161)

Bug 5241: Block to-localhost, to-link-local requests by default (#1161)

Squid suggested blocking to-localhost access since 2001 commit 4cc6eb1.
At that time, the default was not adjusted because some use cases were
known to require to-localhost access. However, the existence of special
cases should not affect defaults! The _default_ configuration should
either block all traffic or only allow traffic that is unlikely to
introduce new attack vectors into the network.

Also block to-link-local traffic (by default), for very similar reasons:
Popular cloud services use well-known IPv4 link-local (i.e. RFC 3927)
addresses (a.k.a. APIPA), to provide sensitive instance metadata
information, via HTTP, to instance users and scripts. Given cloud
popularity, those special addresses become nearly as ubiquitous as
127.0.0.1. Cloud provider networks shield metadata services from
external accesses, but proxies like Squid that forward external HTTP
requests may circumvent that protection.