- Make option spaces stored as linked lists first-class objects.

- Make option spaces stored as linked lists first-class objects.
- Use these spaces for the agent option space, fqdn option space and
  nwip option space.
- Make encapsulator and decoder for fqdn option space.
- Support the agent option space the way all option spaces are supported
  to the extent possibol, rather than special-casing it.
- Add an enumerator function for defined options in an option space.
- Support encapsulated options directly with a format string, rather than
  as another special case.