Consistent handling of by/field arguments for plot.py and summary.py

Now both scripts also fallback to guessing what fields to use based on
what fields can be converted to integers. This is more falible, and
doesn't work for tests/benchmarks, but in those cases explicit fields
can be used (which is what would be needed without guessing anyways).

Makefile

@@ -170,10 +170,11 @@ coverage: $(GCDA)
 .PHONY: summary sizes
 summary sizes: $(BUILDDIR)lfs.csv
 	$(strip ./scripts/summary.py -Y $^ \
-		-f code=code_size,$\
+		-fcode=code_size,$\
 			data=data_size,$\
 			stack=stack_limit,$\
 			struct=struct_size \
+		--max=stack \
 		$(SUMMARYFLAGS))