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_extreme_restriction_threshold(void)
{
  int32_t pct = networkstatus_get_param(NULL,
                                        "guard-extreme-restriction-percent",
                                        DFLT_EXTREME_RESTRICTION_PERCENT,
                                        1, INT32_MAX);
  return pct / 100.0;
}

get_extreme_restriction_threshold(void)
{
  int32_t pct = networkstatus_get_param(NULL,
                                        "guard-extreme-restriction-percent",
                                        DFLT_EXTREME_RESTRICTION_PERCENT,
                                        1, INT32_MAX);
  return pct / 100.0;
}
