Add sockaddr_alg to sock_addr_t (#234)

sock_addr_t is used to define the minimum size of any socket address on
the stack. Let's make sure that an AF_ALG address always fits. Coverity
complains because in theory, AF_ALG might be larger than any of the other
structs. In practice it already fits.

Closes Coverity CID 1398948, 1398949, 1398950

Signed-off-by: Christian Heimes <christian@python.org>