lexically sort vfields in toptable if no order is specified

On perls with hash randomization (>= 5.18) this makes UTs pass and
makes for a nicer user experience (columns don't randomly reorder themselves).

Note: table3/4 in the UTs are reorderings of table2

5.18.1 test run now passes, skipping only fromtcpdump and togdgraph (having issues installing deps for both, so can't confirm or deny).