core: Add 'core squeuestats' as a read-only query

The queryhandler API is seriously under-utilized. With this patch we
augment it somewhat to get some reasonably useful stats out of the
scheduling queue. Events are tracked from "remove_event()" and
"add_event()", which are the only two valid entry-points for the main
scheduling queue. The printed stats is simply a count of how events
scheduled, broken down per type.

Signed-off-by: Andreas Ericsson <ae@op5.se>
core: Modify EVENT_TYPE_STR() macro to yield shorter strings

We don't particularly want to type lots and lots when we grab info
from the query handler, so avoiding the common EVENT_ prefix when
we know we're dealing with event types is just plain good.

Signed-off-by: Andreas Ericsson <ae@op5.se>