IPV6 fix

The client can't connect using ipv6 because net_addr_from_str() is broken. (net_addr_from_str() hasn't been used for connecting in 0.6.*)

inet_pton() is being used wrong:
https://github.com/teeworlds/teeworlds/blob/master/src/base/system.c#L784

man:
int inet_pton(int af, const char *src, void *dst);

"src points to a character string containing an IPv6 network address. The address is converted to a struct in6_addr and copied to dst, which must be sizeof(struct in6_addr) (16) bytes (128 bits) long. The allowed formats for IPv6 addresses follow these rules: [...]"