$NIX_PATH may contain elements that don't evaluate to an attrset (like
"nixos-config"), so ignore those.

BuildError denotes a permanent build failure, which is not the case
here.

Do not try to fill fd_set with fd>=FD_SETSIZE

This is UB and causes buffer overflow and crash on linux.

Update mailing list.

In particular, don't use base-64, which we don't support. (We do have
base-32 redirects for hysterical reasons.)

Also, add a test for the hashed mirror feature.

In particular, this allows it to be disabled in our tests.

use sha256 hashes in the examples

And fix a dead link.

E.g.

  $ nix path-info --json --store https://cache.nixos.org nixpkgs.thunderbird -S
  ...
      "downloadHash": "sha256:1jlixpzi225wwa0f4xdrwrqgi47ip1qpj9p06fyxxg07sfmyi4q0",
      "downloadSize": 43047620,
      "closureDownloadSize": 84745960
    }
  ]

This doesn't work in read-only mode, ensuring that operations like

  nix path-info --store https://cache.nixos.org -S nixpkgs.hello

(asking for the closure size of nixpkgs.hello in cache.nixos.org) work
when nixpkgs.hello doesn't exist in the local store.

Now you get

  [
    {
      "path": "/nix/store/fzvliz4j5xzvnd0w5zgw2l0ksqh578yk-bla",
      "valid": false
    }
  ]

On second though this was annoying. E.g. "nix log nixpkgs.hello" would
build/download Hello first, even though the log can be fetched
directly from the binary cache.

May need to revisit this.

Multi-user installer for Darwin (master edition)

Also output in the status report that the user is very silly

Starve the TTY of input to ensure this works, but provide yes to the
current installer to handle the current broken case.

Don't install a second nix after the initial installation, and the rsync change fixes a bug hidden by the nix replacement where the store files were being owned by the installing user due to rsync's -a implying -og.

relevant directions

Prompt for sudo before validating assumptions, and check ourselves for root-owned files instead of making a scary warning.

Fixes #1464.

Not really necessary anymore for #849, but still nice to have.