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.
compare_guards_by_confirmed_idx(const void **a_, const void **b_)
{
  const entry_guard_t *a = *a_, *b = *b_;
  if (a->confirmed_idx < b->confirmed_idx)
    return -1;
  else if (a->confirmed_idx > b->confirmed_idx)
    return 1;
  else
    return 0;
}

compare_guards_by_confirmed_idx(const void **a_, const void **b_)
{
  const entry_guard_t *a = *a_, *b = *b_;
  if (a->confirmed_idx < b->confirmed_idx)
    return -1;
  else if (a->confirmed_idx > b->confirmed_idx)
    return 1;
  else
    return 0;
}
