We want to be able to inject existing file handles + offsets and
even lengths into this in the future, without going through the
->getline interface[1]
We also switch to using a 64K buffer size since we can safely
discard whatever got truncated on write and full writes can help
negotiate a larger TCP window for high-latency, high-bandwidth
links.
While we're at it, make it obvious that we're using O_APPEND for
our tmpfile() interface so we can seek freely for reading while
the writer always prints to the end of the file.
[1] the getline interface for serving static files may result
in us buffering on-FS data into another temporary file,
which is a waste.