libev: move loop setting and backend printing to event_ops->init()

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