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).

Expands first paragraph a bit
Adds a more comprehensive example

Fixes #937

Commit c2154d4c renamed
"build-use-substitutes" to "use-substitutes", but that broke
"nix-copy-closure --use-substitutes".

Unlike signed-binary-caches (which could only be '*' or ''),
require-sigs is a proper Boolean option. The default is true.

The name had become a misnomer since it's not only for substitution
from binary caches, but when adding/copying any
(non-content-addressed) path to a store.

This didn't support specifying a revision/branch, and was restricted
to git:// URIs (since https:// or ssh:// would be ambiguous).

 + remove trailing whitespace from the file

This allows network access in restricted eval mode.

Accidentally committed this change as part of
f9686885.

Restricted mode != pure mode.

Almost all other primops are camelCase so no reason not to use that
here.

A couple makefiles in the sources have -std=c++14 in the CFLAGS.

Nix can now automatically run the garbage collector during builds or
while adding paths to the store. The option "min-free = <bytes>"
specifies that Nix should run the garbage collector whenever free
space in the Nix store drops below <bytes>. It will then delete
garbage until "max-free" bytes are available.

Garbage collection during builds is asynchronous; running builds are
not paused and new builds are not blocked. However, there also is a
synchronous GC run prior to the first build/substitution.

Currently, no old GC roots are deleted (as in "nix-collect-garbage
-d").

In particular, drop the "build-" and "gc-" prefixes which are
pointless. So now you can say

  nix build --no-sandbox

instead of

  nix build --no-build-use-sandbox

This removes the file nix-mode.el from Nix. The file is now available within the
repository https://github.com/NixOS/nix-mode.

Fixes #662
Fixes #1040
Fixes #1054
Fixes #1055
Closes #1119
Fixes #1419

NOTE: all of the above should be fixed within NixOS/nix-mode. If one of those
hasn’t please reopen within NixOS/nix-mode and not within NixOS/nix.

The function 'builtins.split' takes a POSIX extended regular expression
and an arbitrary string. It returns a list of non-matching substring
interleaved by lists of matched groups of the regular expression.

```nix
with builtins;
assert split "(a)b" "abc"      == [ "" [ "a" ] "c" ];
assert split "([ac])" "abc"    == [ "" [ "a" ] "b" [ "c" ] "" ];
assert split "(a)|(c)" "abc"   == [ "" [ "a" null ] "b" [ null "c" ] "" ];
assert split "([[:upper:]]+)" "  FOO   "
                               == [ "  " [ "FOO" ] "   " ];
```

except in older release notes where the name was actually Mac OS X.

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

And fix a dead link.