The images and containers created by docker_wrapper have a particular form:
boinc__<proj>__<wuname> and boinc__<proj>__<resultname>.

docker_wrapper normally cleans up after itself.
But if it crashes these objects could hang around forever,
taking up disk space.

So we clean them up on client startup, as follows:
make lists of the names for active Docker tasks.
Then go through the list of Docker images and containers.
Remove any that
- were created by Docker wrapper (i.e. have boinc__ names)
- aren't in the active-task lists

On Unix, do this on the host itself if it has Docker installed.
On Win, do it for every WSL distro that has Docker installed.

Make 'verbose' an attribute of DOCKER_CONN.

Maybe we should divide util.cpp into
- things used from the scheduler and file upload handlers
    (where we need boinc::stdio stuff)
- everything else