r23: - Remove support for multiple mappings in vulpes: it's broken, unused, and unnecessary.  (Fauxide doesn't support multiple instances, and for its replacements, we should be using a separate vulpes instance for each virtual disk.)
r23: - Remove support for multiple mappings in vulpes: it's broken, unused,
and unnecessary.  (Fauxide doesn't support multiple instances, and for its
replacements, we should be using a separate vulpes instance for each virtual
disk.)

- For those command-line options we should only be given once, make sure
we actually only get them once.

- Reformat usage message to improve readability.

- Add --help command-line option.

cmusatyalab/openisr,cmusatyalab/openisr,cmusatyalab/openisr,cmusatyalab/openisr,cmusatyalab/openisr