Initial sockets support for the Wii.

Not functional yet due to bugs in the SDK.
Bugs found so far:

 * Binding to port 0 fails instead of choosing a free port.
 * gethostbyname() does not work for IPs.
 * getsockname() is missing.
 * struct sockaddr_storage is missing.

I have not decided yet whether I fix those bugs in the SDK (I already
implemented getsockname() and added struct sockaddr_stroage and it seems
to work) or if I work around them in ObjFW. This will mainly depend on
how cooperative the developers of the Wii SDK are.