fastping.go
func (p *Pinger) AddIPAddr(ip *net.IPAddr) {
	p.mu.Lock()
	p.addrs[ip.String()] = ip
	if isIPv4(ip.IP) {
		p.hasIPv4 = true
	} else if isIPv6(ip.IP) {
		p.hasIPv6 = true
	}
	p.mu.Unlock()
}
func (p *Pinger) AddIPAddr(ip *net.IPAddr) {
	p.mu.Lock()
	p.addrs[ip.String()] = ip
	if isIPv4(ip.IP) {
		p.hasIPv4 = true
	} else if isIPv6(ip.IP) {
		p.hasIPv6 = true
	}
	p.mu.Unlock()
}
// AddIPAddr adds an IP address to Pinger. ip arg should be a net.IPAddr
// pointer.