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

Signed-off-by: Alejandro Mery <alejandro.mery@opensynergy.com>
introduce context->event_ops

Signed-off-by: Alejandro Mery <alejandro.mery@opensynergy.com>
libev: mutilate and prepare for context->event_ops usage

Signed-off-by: Alejandro Mery <alejandro.mery@opensynergy.com>
libev: rename to lib/event-libev.c

Signed-off-by: Alejandro Mery <alejandro.mery@opensynergy.com>
move lws_plat_service() implementations to context->event_ops->service()

Signed-off-by: Alejandro Mery <alejandro.mery@opensynergy.com>
libev: add initial implementation of event_ops->service()

Signed-off-by: Alejandro Mery <alejandro.mery@opensynergy.com>
move register, unregister and watching direction change into event_ops

Signed-off-by: Alejandro Mery <alejandro.mery@opensynergy.com>

Conflicts:
	lib/private-libwebsockets.h
libev: change test-client and test-server to set info->event_ops for -e

Signed-off-by: Alejandro Mery <alejandro.mery@opensynergy.com>
libev: implement socket_register(), socket_unregister() and socket_change()

Signed-off-by: Alejandro Mery <alejandro.mery@opensynergy.com>