SDL3_net returns negative values on failure

[madebr]

A few SDL3_net functions return negative values on failure.
These should probably be changed into returning false on failure.

[icculus]

This turned out to be only a few of the functions.

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.

[madebr]

Indeed, let's create an issue for that so we don't forget about it.

[icculus]

Done!