Currently, libcontainer-based executor, upon shutdown, kills the
container initial process. The children of the killed process remain
running, and the executor is never marked as terminated until they do.
Also, fix a case where we treat processes as successful, when
`proc.Wait()` fails. In some attempts, I was getting "waitid no child
processes" errors and such error shouldn't get process to be considered
successful.
As part of deprecating legacy drivers, we're moving the env package to a
new drivers/shared tree, as it is used by the modern docker and rkt
driver packages, and is useful for 3rd party plugins.
Tests expect that as soon as eventer shuts down immediately on context
cancellations; but golang does not guarantee priority when multiple
pending channels are ready in a select statement.
This PR plumbs the plugins done ctx through the base and driver plugin
clients (device already had it). Further, it adds generic handling of
gRPC stream errors.
