libwebsockets

optimize extpoll fd delete

Previous method of shifting back array by one to cover the deleted
item could be expensive when the list was large and the deleted item
appeared early in it.

This scheme swaps the last guy into the vacant space and reduces the
list size by one.

(AG adapted for style and not to care if n is end guy)

Signed-off-by: Edwin van der Oetelaar <oetelaar.automatisering@gmail.com>
Signed-off-by: Andy Green <andy.green@linaro.org>