libubus: do not modify uloop_cancelled

uloop_cancelled was used for two purposes within ubus_complete_request:
- interrupting recursive requests on SIGINT/SIGTERM
- breaking out of the poll loop in a recursive request that completed

Saving/restorung uloop_cancelled was buggy, leading to SIGTERM not being
processed properly. Simplify the logic by using a separate field for
internal use

Signed-off-by: Felix Fietkau <nbd@nbd.name>