benchmark: add http chunked encoding benchmarks

Test both "Content-Length: xxx" and "Transfer-Encoding: chunked" responses in
the http/simple benchmark.
test: fix async listener test for windows
tls: add ECDH ciphers support

Switch test fixtures to 1024 bit keys.
src: squelch unused variable warning

|i| and |j| arent't used when building without crypto support.  Hat tip
to Brian White.

Rename |l| to |k| while we're here because it's quite hard to discern
from |i| or |j| with some fonts.