SDL client/server: add a Client structure and a NICKNAME command
SDLNet client/server: several improvements, improve the management of clients' names, and the WHO command now send to the client the list of connected clients instead of displaying it server-side
SDLNet client/server: clean of the code, updated README
t4k_common porting: setup.c: add the T4K debugging options in the "Usage" text
t4k_common porting: setup.c: copy handle_debug_args() from tuxmath so that handle_command_args() can now parse the new T4K debugging options
t4k_common porting: add debug_setup debugging option, update setup.c DEBUGCODE and DEBUGMSG calls to be able to use the debug_setup option
setup.c: replace DEBUGCODE with DEBUGMSG whenever possible
main.c: replace DEBUGCODE with DEBUGMSG whenever possible