service: log stdout and stderr to separate files
service: log stdout and stderr to separate files

Previously we would pass the path `/var/log/service.log` for both
stdout and stderr to containerd. containerd would construct a dict
with the paths as keys[1] and, due to the duplicate key, would only
open one of the files and start one `io.Copy` instance. Writes to
the other stream would be buffered by the pipe connected to
containerd-shim and would eventually block.

If we modified containerd to open the file twice and start 2
`io.Copy` instances, we would end up with the two streams interleaved
together. It seems cleaner to keep the streams separate; therefore
this patch logs stdout to `/var/log/service.out.log` and stderr to
`/var/log/service.err.log`.

[1]
https://github.com/containerd/containerd/blob/49437711c31c5f8ca8e1a5931be7284198497350/linux/shim/io.go#L51

Signed-off-by: David Scott <63c9eb0ea83039690fefa11afe17873ba8278a56@docker.com>

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