bpo-38282: Rewrite getsockaddrarg() helper function (GH-16698)

Rewrite getsockaddrarg() helper function of socketmodule.c (_socket
module) to prevent a false alarm when compiling codde using GCC with
_FORTIFY_SOURCE=2. Pass a pointer of the sock_addr_t union, rather
than passing a pointer to a sockaddr structure.

Add "struct sockaddr_tipc tipc;" to the sock_addr_t union.