context: move poll/win32 specifics into a sub-struct

Signed-off-by: Alejandro Mery <amery@geeks.cl>
introduce context->event_ops and the info->event selector

Signed-off-by: Alejandro Mery <amery@geeks.cl>
libev: mutilate and prepare for context->event_ops usage

Signed-off-by: Alejandro Mery <amery@geeks.cl>
libev: rename to lib/event-libev.c

Signed-off-by: Alejandro Mery <amery@geeks.cl>
select context->event_ops based on info->event

Signed-off-by: Alejandro Mery <amery@geeks.cl>
move lws_plat_service() implementations to context->event_ops->service()

Signed-off-by: Alejandro Mery <amery@geeks.cl>
libev: add initial implementation of event_ops->service()

Signed-off-by: Alejandro Mery <amery@geeks.cl>
move register, unregister and watching direction change into event_ops

Signed-off-by: Alejandro Mery <amery@geeks.cl>
libev: change test-client and test-server to set info->event for -e

Signed-off-by: Alejandro Mery <amery@geeks.cl>
libev: also select libev if the LWS_SERVER_OPTION_LIBEV flag is set

Signed-off-by: Alejandro Mery <amery@geeks.cl>
libev: implement socket_register(), socket_unregister() and socket_change()

Signed-off-by: Alejandro Mery <amery@geeks.cl>