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.
mark_primary_guards_maybe_reachable(guard_selection_t *gs)
{
  tor_assert(gs);

  if (!gs->primary_guards_up_to_date)
    entry_guards_update_primary(gs);

  SMARTLIST_FOREACH_BEGIN(gs->primary_entry_guards, entry_guard_t *, guard) {
    mark_guard_maybe_reachable(guard);
  } SMARTLIST_FOREACH_END(guard);
}

mark_primary_guards_maybe_reachable(guard_selection_t *gs)
{
  tor_assert(gs);

  if (!gs->primary_guards_up_to_date)
    entry_guards_update_primary(gs);

  SMARTLIST_FOREACH_BEGIN(gs->primary_entry_guards, entry_guard_t *, guard) {
    mark_guard_maybe_reachable(guard);
  } SMARTLIST_FOREACH_END(guard);
}
