Invert the signature logic of in{,6}_selectsrc, make them return the

Invert the signature logic of in{,6}_selectsrc, make them return the
error code and pass the resulting source address back to the caller
through a pointer, as suggested by chrisz.  This gives us more readable
code, and eases the deletion of useless checks in the callers' error path.
Add a bunch of "0 -> NULL" conversions, while here.
ok chrisz@ mpi@