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.
have_sampled_guard_with_id(guard_selection_t *gs, const uint8_t *rsa_id)
{
  return get_sampled_guard_with_id(gs, rsa_id) != NULL;
}

have_sampled_guard_with_id(guard_selection_t *gs, const uint8_t *rsa_id)
{
  return get_sampled_guard_with_id(gs, rsa_id) != NULL;
}
