net: Define struct sockaddr_storage

POSIX doesn't guarantee that "legacy" struct sockaddr is large enough
for all usages, e.g. IPv6 addresses, and instead requires use of
struct sockaddr_storage:
http://pubs.opengroup.org/onlinepubs/009696699/basedefs/sys/socket.h.html



    ... shall define the sockaddr_storage structure. This structure
    shall be:

    Large enough to accommodate all supported protocol-specific
    address structures

Signed-off-by: Paul Sokolovsky <paul.sokolovsky@linaro.org>