Consider the exit family when applying guard restrictions.

When the new path selection logic went into place, I accidentally
dropped the code that considered the _family_ of the exit node when
deciding if the guard was usable, and we didn't catch that during
code review.

This patch makes the guard_restriction_t code consider the exit
family as well, and adds some (hopefully redundant) checks for the
case where we lack a node_t for a guard but we have a bridge_info_t
for it.

Fixes bug 22753; bugfix on 0.3.0.1-alpha. Tracked as TROVE-2016-006
and CVE-2017-0377.
node_has_ipv6_addr(const node_t *node)
{
  /* Don't check the ORPort or DirPort, as this function isn't port-specific,
   * and the node might have a valid IPv6 address, yet have a zero
   * ORPort or DirPort.
   */
  if (node->ri && tor_addr_is_valid(&node->ri->ipv6_addr, 0))
    return 1;
  if (node->rs && tor_addr_is_valid(&node->rs->ipv6_addr, 0))
    return 1;
  if (node->md && tor_addr_is_valid(&node->md->ipv6_addr, 0))
    return 1;

  return 0;
}

node_has_ipv6_addr(const node_t *node)
{
  /* Don't check the ORPort or DirPort, as this function isn't port-specific,
   * and the node might have a valid IPv6 address, yet have a zero
   * ORPort or DirPort.
   */
  if (node->ri && tor_addr_is_valid(&node->ri->ipv6_addr, 0))
    return 1;
  if (node->rs && tor_addr_is_valid(&node->rs->ipv6_addr, 0))
    return 1;
  if (node->md && tor_addr_is_valid(&node->md->ipv6_addr, 0))
    return 1;

  return 0;
}
