SEGFAULT core dump after detach; ReventLoop::RemovePollHandler() can cause race condition

[wfjm]

The detach of a tcp type virtual terminal or a tap type virtual ethernet device can lead to a SEGFAULT core dump, e.g. after a

    cpu0ttb0 det

This is caused by a race condition between the detach run-down and the implementation of
ReventLoop::RemovePollHandler().

[wfjm]

The culprit was a design error in the RemovePollHandler flow, which allowed that DoPoll returned while poll list updates were still pending and the subsequent execution of DoCall called removed handlers.

DoPoll now loops until there are no pending poll list updates and DoCall quits processing poll change notifications when it detects pending poll list updates.

Fixed with commit 6f56f29.