tri-state return values should be enums, not ints

[icculus]

Most of the things returning int are doing -1==failure, 1==success, 0==wouldblock.

These sort of three-state return codes moved to enums in SDL3 (SDL_AppResult, SDL_EnumResult, etc), and we should maybe do that here, too.

Originally posted by @icculus in https://github.com/libsdl-org/SDL_net/issues/111#issuecomment-2433843616