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.
nodelist_find_nodes_with_microdesc(const microdesc_t *md)
{
  smartlist_t *result = smartlist_new();

  if (the_nodelist == NULL)
    return result;

  SMARTLIST_FOREACH_BEGIN(the_nodelist->nodes, node_t *, node) {
    if (node->md == md) {
      smartlist_add(result, node);
    }
  } SMARTLIST_FOREACH_END(node);

  return result;
}

nodelist_find_nodes_with_microdesc(const microdesc_t *md)
{
  smartlist_t *result = smartlist_new();

  if (the_nodelist == NULL)
    return result;

  SMARTLIST_FOREACH_BEGIN(the_nodelist->nodes, node_t *, node) {
    if (node->md == md) {
      smartlist_add(result, node);
    }
  } SMARTLIST_FOREACH_END(node);

  return result;
}
