This patch is in preparation for the gateway's extraction.

It's interesting to trace technical debt back to its
origin, understanding the circumstances in which it
was introduced and built up, and then cutting it back
at exactly the right places.

- Clean up the gateway's surface
  The option builder GatewayOption() now takes only
  arguments which are relevant for HTTP handler muxing,
  i.e. the paths where the gateway should be mounted.
  All other configuration happens through the
  GatewayConfig object.

- Remove BlockList
  I know why this was introduced in the first place,
  but it never ended up fulfilling that purpose.
  Somehow it was only ever used by the API server,
  not the gateway, which really doesn't make sense.
  It was also never wired up with CLI nor fs-repo.
  Eventually @krl started punching holes into it
  to make the Web UI accessible.

- Remove --unrestricted-api
  This was holes being punched into BlockList too,
  for accessing /ipfs and /ipn on the API server.
  With BlockList removed and /ipfs and /ipns freely
  accessible, putting this option out of action
  is safe. With the next major release,
  the option can be removed for good.

License: MIT
Signed-off-by: Lars Gierth <larsg@systemli.org>