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_get_mutable_by_id(const char *identity_digest)
{
  node_t search, *node;
  if (PREDICT_UNLIKELY(the_nodelist == NULL))
    return NULL;

  memcpy(&search.identity, identity_digest, DIGEST_LEN);
  node = HT_FIND(nodelist_map, &the_nodelist->nodes_by_id, &search);
  return node;
}

node_get_mutable_by_id(const char *identity_digest)
{
  node_t search, *node;
  if (PREDICT_UNLIKELY(the_nodelist == NULL))
    return NULL;

  memcpy(&search.identity, identity_digest, DIGEST_LEN);
  node = HT_FIND(nodelist_map, &the_nodelist->nodes_by_id, &search);
  return node;
}
