libwebsockets

merge test server extpoll into test server

the -extpoll version of the test server was starting to rot compared to
the test-server.c it was originally based on.

This patch deletes the -extpoll.c version and instead has the test-server.c
source built two different ways in the makefile, once with the define
EXTERNAL_POLL which forces non-fork mode and enables the "by hand"
pollfd array handling.  The resulting binary of that is still called
libwebsockets-test-server-extpoll.

Another problem was that the pollfd array length needs to match MAX_CLIENTS, that
now happens during the build.

Signed-off-by: Andy Green <andy.green@linaro.org>