Modernize LoadSavedGame() in file.c & ./include/file.h

[de01dwc]

• replace sprintf() calls with snprintf()
• char array size set with hard-coded value update 80 to const expression with relevant name
• when construction save file, move saveFile assignment to its own line for readability, then check for NULL
• change fopen param to 'r' instead of 'r+' since the file is only being read
• remove unnecessary cast in first param of fwrite()
• update fclose() to check for EOF or != 0 instead of < 0
• cast to u_long; remove cast if possible, or update to correct type
• update if(condition == FALSE) to if(!condition)
• possible missing 'return false;' after failed ReadNextLevel() call, ~line 209