add locking callback for fds

This adds two new callbacks in protocols[0] that are optional for allowing limited thread
access to libwebsockets, LWS_CALLBACK_LOCK_POLL and LWS_CALLBACK_UNLOCK_POLL.

If you use them, they protect internal and external poll list changes, but if you want to use
external thread access to libwebsocket_callback_on_writable() you have to implement your
locking here even if you don't use external poll support.

If you will use another thread for this, take a lot of care about managing your list of
live wsi by doing it from ESTABLISHED and CLOSED callbacks (with your own locking).

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