exit_with_usage() return 1 in error case only

"screen -h|--help" should NOT return 1

src/help.c

@@ -111,8 +111,9 @@ void exit_with_usage(char *myname, char *message, char *arg)
     printf("\nError: ");
     printf(message, arg);
     printf("\n");
+    exit(1);
   }
-  exit(1);
+  exit(0);
 }
 
 /* Here come the help page routines */