datatype: avoid cast-align warning with struct sockaddr result from getaddrinfo()

datatype: avoid cast-align warning with struct sockaddr result from getaddrinfo()

With CC=clang we get

    datatype.c:625:11: error: cast from 'struct sockaddr *' to 'struct sockaddr_in *' increases required alignment from 2 to 4 [-Werror,-Wcast-align]
                    addr = ((struct sockaddr_in *)ai->ai_addr)->sin_addr;
                            ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    datatype.c:690:11: error: cast from 'struct sockaddr *' to 'struct sockaddr_in6 *' increases required alignment from 2 to 4 [-Werror,-Wcast-align]
                    addr = ((struct sockaddr_in6 *)ai->ai_addr)->sin6_addr;
                            ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    datatype.c:826:11: error: cast from 'struct sockaddr *' to 'struct sockaddr_in *' increases required alignment from 2 to 4 [-Werror,-Wcast-align]
                    port = ((struct sockaddr_in *)ai->ai_addr)->sin_port;
                            ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Fix that by casting to (void*) first. Also, add an assertion that the
type is as expected.

For inet_service_type_parse(), differentiate between AF_INET and
AF_INET6. It might not have been a problem in practice, because the
struct offsets of sin_port/sin6_port are identical.

Signed-off-by: Thomas Haller <thaller@redhat.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>