Combined with the previous changes, stack traces involving derivations
are now much less verbose, since something like

  while evaluating the builtin function `getAttr':
  while evaluating the builtin function `derivationStrict':
  while instantiating the derivation named `gtk+-2.24.20' at `/home/eelco/Dev/nixpkgs/pkgs/development/libraries/gtk+/2.x.nix:11:3':
  while evaluating the derivation attribute `propagatedNativeBuildInputs' at `/home/eelco/Dev/nixpkgs/pkgs/stdenv/generic/default.nix:78:17':
  while evaluating the attribute `outPath' at `/nix/store/212ngf4ph63mp6p1np2bapkfikpakfv7-nix-1.6/share/nix/corepkgs/derivation.nix:18:9':
  ...

now reads

  while evaluating the attribute `propagatedNativeBuildInputs' of the derivation `gtk+-2.24.20' at `/home/eelco/Dev/nixpkgs/pkgs/development/libraries/gtk+/2.x.nix:11:3':
  ...

This is equivalent to running ‘nix-env -e '*'’ first, except that it
happens in a single transaction.  Thus, ‘nix-env -i pkgs...’ replaces
the profile with the specified set of packages.

The main motivation is to support declarative package management
(similar to environment.systemPackages in NixOS).  That is, if you
have a specification ‘profile.nix’ like this:

  with import <nixpkgs> {};
  [ thunderbird
    geeqie
    ...
  ]

then after any change to ‘profile.nix’, you can run:

  $ nix-env -f profile.nix -ir

to update the profile to match the specification.  (Without the ‘-r’
flag, if you remove a package from ‘profile.nix’, it won't be removed
from the actual profile.)

Suggested by @zefhemel.

Issue #162.

Reported by Matija Šuklje.

Fixes #163.

This is mostly useful for Hydra to deal with builders that get stuck
in an infinite loop writing data to stdout/stderr.

These cause an unnecessary runtime dependency :-)

Libpng used to accept this, but no longer does.

This causes the environment to be (almost) cleared, thus giving a
shell that more closely resembled the actual Nix derivation.

This ensures that not just environment variables are set, but also
shell functions such as unpackPhase, configurePhase and so on.

Issue NixOS/hydra#102.

This allows providing additional binary caches, useful in scripts like
Hydra's build reproduction scripts, in particular because untrusted
caches are ignored.

Reported by Johan Grande.

This reverts commit 28bba8c4.

Also use a point release version number as suggested by several
people.