The inner lambda was returning a SQLite-internal char * rather than a
std::string, leading to Hydra errors liks

  Caught exception in Hydra::Controller::Root->narinfo "path ‘ø˜£â€™ is not in the Nix store at /nix/store/6mvvyb8fgwj23miyal5mdr8ik4ixk15w-hydra-0.1.1234.abcdef/libexec/hydra/lib/Hydra/Controller/Root.pm line 352."

That is, unless --file is specified, the Nix search path is
synthesized into an attribute set. Thus you can say

  $ nix build nixpkgs.hello

assuming $NIX_PATH contains an entry of the form "nixpkgs=...". This
is more verbose than

  $ nix build hello

but is less ambiguous.

For example, you can now say:

  configureFlags = "--prefix=${placeholder "out"} --includedir=${placeholder "dev"}";

The strings returned by the ‘placeholder’ builtin are replaced at
build time by the actual store paths corresponding to the specified
outputs.

Previously, you had to work around the inability to self-reference by doing stuff like:

  preConfigure = ''
    configureFlags+=" --prefix $out --includedir=$dev"
  '';

or rely on ad-hoc variable interpolation semantics in Autoconf or Make
(e.g. --prefix=\$(out)), which doesn't always work.

Allow contexted strings in replaceStrings

Otherwise in particular https://cache.nixos.org won't work in the
daemon.

This was only used by nix-push.

Let's step in line with time and document more realistic values.

This makes us more robust against 500 errors from CloudFront or S3
(assuming the 500 error isn't cached by CloudFront...).

This allows such paths to be imported without signatures.

This makes it easier to create a diverted store, i.e.

  NIX_REMOTE="local?root=/tmp/root"

instead of

  NIX_REMOTE="local?real=/tmp/root/nix/store&state=/tmp/root/nix/var/nix" NIX_LOG_DIR=/tmp/root/nix/var/log

This variable has no reason to exist, given $NIX_STATE_DIR.

Nuke nix-push.

Rarely used, nix copy replaces it.

We have BinaryCacheStore now

Was added in 2006 to "measure the cost of the Nix approach".

Given that it uses /usr/bin/perl, I think this is safe to remove.

Remove nix-install-package.

Refs #831

* Dockerfile: add GNU tar native dependency

`builtins.fetchTarball` requires GNU tar to be available in the $PATH in
order to unpack the fetched tarball (there is a FIXME comment for this),
which Alpine does not ship by default (it ships BusyBox tar).

* Dockerfile: add GNU bash native dependency

`nix-shell` defaults to invoking `bash` from the $PATH for the subshell.
In theory this can be overriden with the NIX_BUILD_SHELL environment
variable, but this does not work properly. `nix-shell` generates and
passes a script (`$rcFile`) to be executed by the subshell which uses
bashisms (`source` and `shopt`). Additionally, in interactive mode,
`nix-shell` passes the `--rcfile` argument to the shell, which is
another bashism.

Because `bash` is thus de-facto required, add `bash` as a native package
dependency to make it available for `nix-shell`.

Currently, man has issues finding man pages for Nix-installed
application (also, `nix-env --help` doesn't work). The issue is caused
by custom `$MANPATH` set by my system. That makes man use it instead of
searching in default location.

Either of next lines workaround the issue:
```sh
unset MANPATH

export MANPATH=$HOME/.nix-profile/share/man:$MANPATH
```

This patch adds the later line to the `nix-profile.sh` if user has
`MANPATH` set. (Not clearing `MANPATH` as that would be disrespect of
user's preferences.)

As a side-effect, host's man might find man pages installed by Nix.

Also, allow builtins.{fetchurl,fetchTarball} in restricted mode if a
hash is specified.

Fixes #977