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.
num_reachable_filtered_guards(guard_selection_t *gs,
                              const entry_guard_restriction_t *rst)
{
  int n_reachable_filtered_guards = 0;
  SMARTLIST_FOREACH_BEGIN(gs->sampled_entry_guards, entry_guard_t *, guard) {
    entry_guard_consider_retry(guard);
    if (! entry_guard_obeys_restriction(guard, rst))
      continue;
    if (guard->is_usable_filtered_guard)
      ++n_reachable_filtered_guards;
  } SMARTLIST_FOREACH_END(guard);
  return n_reachable_filtered_guards;
}

num_reachable_filtered_guards(guard_selection_t *gs,
                              const entry_guard_restriction_t *rst)
{
  int n_reachable_filtered_guards = 0;
  SMARTLIST_FOREACH_BEGIN(gs->sampled_entry_guards, entry_guard_t *, guard) {
    entry_guard_consider_retry(guard);
    if (! entry_guard_obeys_restriction(guard, rst))
      continue;
    if (guard->is_usable_filtered_guard)
      ++n_reachable_filtered_guards;
  } SMARTLIST_FOREACH_END(guard);
  return n_reachable_filtered_guards;
}
