Compatibility with OSes where sin_len has to be set

[vlm]

...otherwise valgrind complains on newer OSes and older OSes might not work correctly.

The provisions for checking the (SA_LEN) were there in the code (line https://github.com/wahern/dns/pull/21/files#diff-b2ddf457bc423779995466f7d8b9d147L4432), I just applied them consistently across the board.