libwebsockets

testapp: extpoll: add back missing EXTERNAL_POLL pieces

EXTERNAL_POLL is not recommended for use for a while, it's a hack to allow
integration of lws with random application poll() loops.

While lws is very happy to do that secondary job for any event lib using the
foreign loop support (for uv, event, glib, and ev), for random roll-your-
own poll() waits there's no api because there's no event lib.  The solution
with a future is upgrade your application to use an event loop.

The test app that supports EXTERNAL_POLL was broken in Apr 2018, so it's
apparently good news nobody has been using it in new implementations
since then.  This patch adds in the missing pieces so we can test it until
it is formally deprecated.