Instant messaging queue and currentlyBuilding commands should report a count of items

I often use the IRC plugin to ask our Jenkins server whether it is busy (so I can e.g. restart it to apply updates, or estimate how soon I would be able to do so), since this is faster than requesting and rendering the HTML page.

Responses over IRC come slowly one line at a time (one or very few lines per second), possibly due to server throttling, so it would be very convenient to have the first line of the response report right away how many items there would be. Often this number is all the information I need.