The "16 KB" block triggers based on how much data passes through the
connection, not on what that data actually is. Testing for it usually
means pointing at a page you know is big enough to download - but that
only works if your target actually has one.
Adds CURL_PAD_KB: set it to a number and every request gets padded
with that many KB of junk data, so the block can be triggered against
any target, even a bare server with nothing to download. Off by
default, and doesn't change anything unless you turn it on.
The padding is written to a temp file and attached from there, not
built inline: a large inline value can hit low, easy-to-reach limits
on some platforms (Cygwin among them).
