uloop: rework event processing, fix use-after-free issues

Recursive calls to uloop_run() need to process already fetched events
first, before running kqueue/epoll to get more.

The state of cur_fd/cur_nfds and the event list needs to be maintained
properly to prevent accidental running of events pointing at deleted
uloop_fd structs.

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