No way to abort enet_host_service

[comex]

In various cases I want to want to send a message over an ENetHost or destroy it while another thread is waiting in enet_host_service. Currently this requires waiting for the timeout to expire, which is a waste of time and requires setting a low timeout, which wastes CPU. It would be better to provide a way to abort it, such as having a local pipe/socket that is also selected on.