r1841: Fix for #1606, can't launch dos exe's.

r1841: Fix for #1606, can't launch dos exe's.
2 related problems - 1). DOS uses chained commands - when we
are replying with sendfile we neglect to send the chained header. 2). Win9x and
DOS TCP stacks blow up when getting data back from a Linux sendfile - "The
engines canna take the strain cap'n". Don't use sendfile for anything less than NT1.
Jeremy.
(This used to be commit 3cd88f48afa589a189bb728d2d2660d058203d4d)