[patch] Correct configure test for sin_len to compile on Tru64 Unix

[Petter Reinholdtsen]

[Wayne Davison]
> The problem is deeper than that.  The HAVE_SOCKADDR_LEN define surrounds
> both code that uses sin_len and sa_len, so your change will probably
> cause problems for other systems.  So, it looks like we need 2 configure
> tests and separate defines for sa_len and sin_len.

Oh.  I searched for sa_len and was unable to find code using it.  But
I only searched in the top level directory.

Two different defines is probably a good idea, to avoid confusion. :)