needed for inter-child rsync.

Saves an import, pain in the ass to type all the time.

Makes it easier to match up reads/writes across log files by using
message byte count

The last time I tested set_nonblock() as a fix for the rsync hang, I
used F_SETFD rather than F_SETFL, which resulted in no error, but also
did not set O_NONBLOCK. Turns out missing O_NONBLOCK was the problem.

The rsync hang was due to every context blocking in os.write() waiting
for either a parent or child buffer to empty, which was exacerbated by
rsync's own pipelining, that allows writes from both sides to proceed
even while reads aren't progressing. The hang was due to os.write() on a
blocking fd blocking until buffer space is available to complete the
write. Partial writes are only supported when O_NONBLOCK is enabled.

Previously _output_buf was racy. This may or may not be cheaper than
simply using a lock, but it requires much less code, so I prefer it for
now.