All plugins in plugin-files will be dlopened, allowing them to
statically construct instances of the various Register* types Nix
supports.

See:
https://github.com/NixOS/nix/commit/88b5d0c8e89afefbc547b6243c5aa5a3ec8176e9#commitcomment-27406365

All ANSI sequences except color setting are now filtered out. In
particular, terminal resets (such as from NixOS VM tests) are filtered
out.

Also, fix the completely broken tab character handling.

Add path primop.

builtins.path allows specifying the name of a path (which makes paths
with store-illegal names now addable), allows adding paths with flat
instead of recursive hashes, allows specifying a filter (so is a
generalization of filterSource), and allows specifying an expected
hash (enabling safe path adding in pure mode).

Fixes #1834.

This makes import-from-derivation work in restricted mode again.

This makes e.g. 'fetchGit ./.' work (assuming that ./. is an allowed
path).

Say "copying" instead of "fetching" when copying from another local
store. Nice for nixos-install.

This is needed by nixos-install, which uses the Nix store on the
installation CD as a substituter. We don't want to disable signature
checking entirely because substitutes from cache.nixos.org should
still be checked. So now we can pas "local?trusted=1" to mark only the
Nix store in /nix as not requiring signatures.

Fixes #1819.

Instead, if a fixed-output derivation produces has an incorrect output
hash, we now unconditionally move the outputs to the path
corresponding with the actual hash and register it as valid. Thus,
after correcting the hash in the Nix expression (e.g. in a fetchurl
call), the fixed-output derivation doesn't have to be built again.

It would still be good to have a command for reporting the actual hash
of a fixed-output derivation (instead of throwing an error), but
"nix-build --hash" didn't do that.

Note that nix-build always creates indirect roots.

Fixes #1830.

Closes #323.

remote_store: register for NIX_REMOTE=unix://path

Following discussion with Shea and Graham. It's a big enough change
from the last release. Also, from a semver perspective, 2.0 makes more
sense because we did remove some interfaces (like nix-pull/nix-push).

This command upgrades Nix to the latest stable version by installing a
store path obtained from

  https://github.com/NixOS/nixpkgs/raw/master/nixos/modules/installer/tools/nix-fallback-paths.nix

which is the same store path that the installer at
https://nixos.org/nix/install.sh uses.

The upgrade fails if Nix is not installed in a profile (e.g. on NixOS,
or when installed outside of the Nix store).

Fix macOS installation when umask disallow public read (solves #1582)

The latter breaks in the case of cross-compilation, when `cpp` bears a
target prefix.

Expands first paragraph a bit
Adds a more comprehensive example

This allows overriding the socket path so the daemon may be listening at
an arbitrary Unix domain socket location.

Fixes #1800

Some servers, such as Artifactory, allow uploading with PUT and BASIC
auth. This allows nix copy to work to upload binaries to those
servers.

Worked on together with @adelbertc

This was for backward compatibility. However, with security-related
configuration settings, it's best not to have any confusion.

Issue #495.

Fix tests using user namespaces on kernels that don't have it