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