Fix function prototype warnings for C23 compatibility

- Add proper function prototypes: fatal(), my_exit(), set_order()
- Fix function pointer types to match call sites: void (*func)(int)
- Remove conflicting localtime() declaration in rip.c
- Fix incorrect new_item() call in state.c (remove unused argument)

Result: Zero compilation warnings, C23 compatible.

See MODERNIZATION_LOG.md for detailed analysis.

rip.c

@@ -230,7 +230,6 @@ death(char monst)
     char **dp, *killer;
     struct tm *lt;
     static time_t date;
-    struct tm *localtime();
 
     signal(SIGINT, SIG_IGN);
     purse -= purse / 10;