Give dissector_all_heur_tables_foreach_table() a sort function.

This makes it a bit more like dissector_all_tables_foreach_table.

Improve comments and clean up whitespace while we're at it.

Change-Id: I5147427f864add285e3bb6cb35ad9fa83bea516c
Reviewed-on: https://code.wireshark.org/review/5714
Reviewed-by: Guy Harris <guy@alum.mit.edu>