libs, bgpd: improve task cancellation by argument value

libs, bgpd: improve task cancellation by argument value

Extend the thread_cancel_event api so that it's more complete:
look in all the lists of events, including io and timers, for
matching tasks. Add a limited version of the api that only
examines tasks in the event and ready queues.

BGP appears to require the old behavior, so change its macro
to use the more limited cancel api.

Signed-off-by: Mark Stapp <mjs@voltanet.io>