web: Get upper limit from /proc/meminfo for container resource sliders

Fixes #390
Closes #636

Signed-off-by: Stef Walter <stefw@redhat.com>
 - Rebased and fixed conflict
cockpit: Make ECONNREFUSED also mean "not-found".

We get this error for example when docker is not running, and
previously we showed it as "internal-error".

Closes #639

Reviewed-by: Stef Walter <stefw@redhat.com>
web: Factor out the DBusClient and DockerClient object caches.

We will use pattern also for the upcoming NetworkManager code.

Closes #628
web: Make "select" buttons inline.

Otherwise the dropdown isn't always under the button.
web: Add DBusInterface.call_with_args
web: Simple network configuration.