Resolve confusing method and macro rebrands

[acerspyro]

Some methods and macros are simply rebrands of readily-available system methods, here are some examples:

ENET_SOCKETSET_ADD -> FD_SET
ENET_SOCKETSET_CHECK -> FD_ISSET

These could be renamed to ENET_SOCKET_FD_SET and ENET_SOCKET_FD_ISSET for example to induce the user into thinking "This is just a wrapper to already existing UNIX methods that I already know about"