of ambiguous nullptr, eliminate use of brace initializer lists

- single global mutex (simpler, easier to make correct for now)
- properly flag kick state for workers to avoid missing wakeups (if
signal is called before wait on the cv)

- make kick_poller() do something on POSIX
- fix some conditions whereby alarms are held in a pollset exec context for too long
- make channel_connectivity tests dependent on the correct behavior