For every peer expose through `getpeerinfo` RPC whether or not we selected them as HB peers, and whether or not they selected us as HB peers.

Suggestion by @gmaxwell.
{
    if (net == NET_UNROUTABLE || net == NET_INTERNAL)
        return;
    LOCK(cs_mapLocalHost);
    vfLimited[net] = !reachable;
}
bool IsReachable(enum Network net)
{
    LOCK(cs_mapLocalHost);
    return !vfLimited[net];
}
bool IsReachable(const CNetAddr &addr)