eThread fixups: Remove "sendcancel" and make sync private

Warn when someone forgot to call kill (which should have been named
join).
Make 'sync' private now that it is unused externally.
Remove the "sendcancel" argument from kill, and move its functionality
into another method called abort_badly().

eHttpStream is the only class to ever call abort_badly, which has been
named "abort_badly" for good reasons (it will leak resources).