Allow external ENet support and small tweak for OpenBSD

Hi --

I'm an OpenBSD dev looking to port mana client and server (I already maintain the manaplus client for OpenBSD).

Here's a patch for manaserv that allows you to build with an ENet already installed on the system. On OpenBSD, we  prefer this to building internal libs, to reduce the amount of work needed to be done.

Also a small tweak for OpenBSD, as we place config files in /etc and not in the directory the binary is located in (which is /usr/local/bin).