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.
get_frac_paths_needed_for_circs(const or_options_t *options,
                                const networkstatus_t *ns)
{
#define DFLT_PCT_USABLE_NEEDED 60
  if (options->PathsNeededToBuildCircuits >= 0.0) {
    return options->PathsNeededToBuildCircuits;
  } else {
    return networkstatus_get_param(ns, "min_paths_for_circs_pct",
                                   DFLT_PCT_USABLE_NEEDED,
                                   25, 95)/100.0;
  }
}

get_frac_paths_needed_for_circs(const or_options_t *options,
                                const networkstatus_t *ns)
{
#define DFLT_PCT_USABLE_NEEDED 60
  if (options->PathsNeededToBuildCircuits >= 0.0) {
    return options->PathsNeededToBuildCircuits;
  } else {
    return networkstatus_get_param(ns, "min_paths_for_circs_pct",
                                   DFLT_PCT_USABLE_NEEDED,
                                   25, 95)/100.0;
  }
}
