struct addrinfo needs padding on Solaris/SPARC

64-bit Solaris/SPARC has a 4-byte pad before ai_addrlen for historical
reasons (ai_addrlen used to be defined as a size_t, which is 4 bytes in
ILP32 and 8 in LP64, but was converted to a 4-byte socklen_t in Solaris
10, which necessitated padding for (more or less) binary compatibility).