web: Add $cockpit.debugging control knob

$cockpit.debugging can be set from the javascript console to one of
these values:

 * 'rest'
 * 'channel'
 * 'dbus'

... to see the relevant debugging logs. Or set to 'all' to see
everything.

Set cookie like this to see debugging information during the loading
of the page:

document.cookie = "debugging=all;" + document.cookie

Reviewed-by: Marius Vollmer <marius.vollmer@redhat.com>
web: Add 'resource' debugging knob for $cockpit.debugging

This will show cgroup monitoring samples in the javascript
console as they come in.

Reviewed-by: Marius Vollmer <marius.vollmer@redhat.com>
web: Cleanup cockpit-docker.js javascript console debugging

Warnings go to console.warn(). Events are already logged via
rest_debug() and remove some other noise.

Reviewed-by: Marius Vollmer <marius.vollmer@redhat.com>