sdl_net: rename back to SDL_net

sdl_net: rename back to SDL_net

It turns out that renaming SDL_net to sdl_net (688c388ce765984af2) isn't
such a good idea, as the name is also used for the tarball filename to
download.

Instead go back to use SDL_net again, and simply add an sdl_net alias.

Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>