Adds IPv6 link local address based communication to libmill

ip.c

/* Convert literal IPv6 address to a binary one. */
static ipaddr mill_ipv6_literal(const char *addr, int port) {
    ipaddr raddr;
    struct sockaddr_in6 *ipv6 = (struct sockaddr_in6*)&raddr;
    struct sockaddr_in6 *sockv6;
    char str[IPADDR_MAXSTRLEN];
    int rc = inet_pton(AF_INET6, addr, &ipv6->sin6_addr);
    mill_assert(rc >= 0);
    if(rc == 1) {
        ipv6->sin6_family = AF_INET6;
        ipv6->sin6_port = htons((uint16_t)port);
    /* Grab IPv6 Scope ID from the IPv6 address. */
    struct ifaddrs *ifaces = NULL;
    int rc = getifaddrs (&ifaces);
    mill_assert (rc == 0);
    mill_assert (ifaces);
    struct ifaddrs *ifv6 = NULL;
    struct ifaddrs *it;
    for(it = ifaces; it != NULL; it = it->ifa_next) {
        if(!it->ifa_addr)
            continue;
	if(it->ifa_addr->sa_family == AF_INET6){
		sockv6 = (struct sockaddr_in6 *)it->ifa_addr;
		inet_ntop(AF_INET6, &(sockv6->sin6_addr), str, IPADDR_MAXSTRLEN);
		if(strcmp(str, addr) == 0){
	    		ipv6->sin6_scope_id = if_nametoindex(it->ifa_name);
	    	}
	}
    }
        errno = 0;
        return raddr;
    }
    ipv6->sin6_family = AF_UNSPEC;
    errno = EINVAL;
    return raddr;
}