Keep source sockaddr for every cached DNS record

Keep source sockaddr for every cached DNS record

This will be required for sending questions on TTL timeouts. When that
happens we want to send unicast question but it's currently broken as we
don't know original IP address.

This change stores whole sockaddr (using struct sockaddr_storage). In
theory it'd be sufficient to store struct in_addr or struct in6_addr but
1) There isn't helper struct for storing IP version agnostic address
2) interface_send_packet already expects struct sockaddr

It hopefully won't hurt memory usage that bad to store a bit more info.

Signed-off-by: Rafał Miłecki <rafal@milecki.pl>