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.
live_consensus_is_missing(const guard_selection_t *gs)
{
  tor_assert(gs);
  if (gs->type == GS_TYPE_BRIDGE) {
    /* We don't update bridges from the consensus; they aren't there. */
    return 0;
  }
  return networkstatus_get_live_consensus(approx_time()) == NULL;
}

live_consensus_is_missing(const guard_selection_t *gs)
{
  tor_assert(gs);
  if (gs->type == GS_TYPE_BRIDGE) {
    /* We don't update bridges from the consensus; they aren't there. */
    return 0;
  }
  return networkstatus_get_live_consensus(approx_time()) == NULL;
}
