StunClient code cleanup: use enet functions, endianness

Instead of using platform-specific sockets, use enet_socket* functions (which ends up doing the same).
Clean up some confusing APIs, removing the distinction between finding the public IP for the host/join.

Fix endianness support & use simpler code.

Refs D364 / rP19703 (and some subsequent fixing diffs).

Differential Revision: https://code.wildfiregames.com/D3970