Add .github/FUNDING.yml

release-common: fix build with `config.allowAliases = false`

```sh
> nix build -f release.nix build.x86_64-linux
error: undefined variable 'docbook5_xsl' at /vcs/nix/release-common.nix:45:7
```

Replace `type` with `command -v` in install script

In POSIX sh, `type` is undefined.

cf. https://pubs.opengroup.org/onlinepubs/9699919799/utilities/command.html#tag_20_22_04

cpptoml now parses almost all examples from the spec.

Turns out we were mis-parsing single-quoted attributes, e.g. 'key2'.

doc: clarify that optional attrs in a function argument will be ignored unless specified

In `args@{ a ? 1 }: /* ... */` the value `a` won't be a part of `args`
unless it's specified when calling the function, the default value will
be ignored in this case.

My personal point of view is that this behavior is a matter of taste, at
least I was pretty sure that unmatched arguments will be a part of
`args@` while debugging some Nix code last week.

I decided to add a warning to the docs which hopefully reduces the
confusion of further Nix developers who thought the same about `args@`.

progress-bar: hide expected if expected is 0 (unknown)

Sync NIX_PROFILES between single-user and multi-user modes

(cherry picked from commit cbfdea68)

See commit 1bffd83e

This is now autodetected. There is no need to put it in the profile.

When we are in single user mode, we still want to have access to
profiles. This way things in Nixpkgs that rely on them getting set
accurately are done in both cases. The point where I hit this is with
using aspell which looks in NIX_PROFILES:

https://github.com/NixOS/nixpkgs/blob/master/pkgs/development/libraries/aspell/default.nix

Before this patch, NIX_PROFILES was never set in single user mode!
This corrects that.

nix: Add --print-build-logs flag

This causes 'nix' to print build log output to stderr rather than
showing the last log line in the progress bar. Log lines are prefixed
by the name of the derivation (minus the version string), e.g.

  binutils> make[1]: Leaving directory '/build/binutils-2.31.1'
  binutils-wrapper> unpacking sources
  binutils-wrapper> patching sources
  ...
  binutils-wrapper> Using dynamic linker: '/nix/store/kr51dlsj9v5cr4n8700jliyz8v5b2q7q-bootstrap-stage0-glibc/lib/ld-linux-x86-64.so.2'
  bootstrap-stage2-gcc-wrapper> unpacking sources
  ...
  linux-headers> unpacking sources
  linux-headers> unpacking source archive /nix/store/8javli69jhj3bkql2c35gsj5vl91p382-linux-4.19.16.tar.xz

Fixes #2709.

https://hydra.nixos.org/build/93359951

build: move needsHashRewrite initialization to startBuilder

build: run diff-hook under --check and document diff-hook

The value of useChroot is not set yet in the constructor, resulting in
hash rewriting being enabled in certain cases where it should not be.

Fixes #2801

Sometimes, "expected" can be "0", but in fact means "unknown".

This is for example the case when downloading a file while the http
server doesn't send the `Content-Length` header, like when running `nix
build` pointing to a nixpkgs checkout streamed from GitHub:

⇒  nix build -f https://github.com/NixOS/nixpkgs/archive/master.tar.gz hello
[1.8/0.0 MiB DL] downloading 'https://github.com/NixOS/nixpkgs/archive/master.tar.gz'

In that case, don't show that weird progress bar, but only the (slowly
increasing) downloaded size ("done").

⇒  nix build -f https://github.com/NixOS/nixpkgs/archive/master.tar.gz hello
[1.8 MiB DL] downloading 'https://github.com/NixOS/nixpkgs/archive/master.tar.gz'

This commit also updates fmt calls with three numbers (when something is
currently 'running' too) - I'm not sure if this can be provoked, but
showing "0" as expected doesn't make any sense, as we're obviously doing
more than nothing.

This prevents spurious syscalls like

  25011 lstat("/home/eelco/with import <nixpkgs> { }; (pkgs.runCommandCC or pkgs.runCommand) \"shell\" { buildInputs = [ (hello) ]; } \"\"", 0x7ffe9c67f580) = -1 ENOENT (No such file or directory)

installer: update macOS version check to 10.12.2

manual: "Nix Package collection" -> "Nixpkgs package collection".