libev: activate libev using info->event instead of LWS_SERVER_OPTION_LIBEV

compatiblity with LWS_SERVER_OPTION_LIBEV is preserved by turning the
flag into info->event = LWS_EVENT_LIBEV when creating the context.

Signed-off-by: Alejandro Mery <amery@geeks.cl>
Adapt test-server and test-client to use libev event backend when passing -e

Signed-off-by: Alejandro Mery <amery@geeks.cl>
libev: don't start sigint and accept watchers unless we are servers

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>