Bug 3478: Allow peer selection

Bug 3478: Allow peer selection

This re-enables Squid peer selection algorithms for intercepted
traffic which has failed Host header verification.

When host verification fails Squid will use, in order of preference:
 * an already PINNED server connection
 * the client ORIGINAL_DST details
 * cache_peer as chosen by selection algorithms

NOTE: whenever DIRECT is selected by routing algorithms the
      ORIGINAL_DST is used instead.

Peer selection results are updated to display PINNED and
ORIGINAL_DST alongside DIRECT and cache_peer.

SECURITY NOTE:

  At this point Squid will pass the request to cache_peer using the
  non-trusted Host header in their URLs. Meaning that the peers
  may still be poisoned by CVE-2009-0801 attacks. Only the initial
  intercepting proxy is protected.

  Full protection against CVE-2009-0801 can be enjoyed by building
  Squid with the -DSTRICT_HOST_VERIFY compile-time flag. This will
  make the peers unreachable for intercepted traffic where the
  Host verification has failed.