The OCI image specification includes a `StopSignal` field in the image
configuration, denoting the system call signal to be sent to the
container to exit. This commit adds a new `WithImageStopSignal` container
option that can be used for storing the `StopSignal` field as a label on
the container. This commit also adjusts `ctr run` to call
`WithImageStopSignal` and `ctr tasks kill` to send the signal stored in
that label by default.
Signed-off-by: Samuel Karp <skarp@amazon.com>
