Mention the --max-freed option in the nix-collect-garbage man page.
Related to https://github.com/NixOS/nix/issues/609.

propagate NIX_BUILD_SHELL also in pure builds document NIX_BUILD_SHELL in the nix-shell command documentation

It's not a limitation of `/usr/bin/env`, it's just how the OS processes shebang lines
(see http://stackoverflow.com/a/4304187)

The text is just a conversion of comment from src/libexpr/primops.cc

Some benchmarking suggested this as a good value. Running

  $ benchmark -f ... -t 25 -- sh -c 'rm -f /nix/var/nix/binary-cache*; nix-store -r /nix/store/x5z8a2yvz8h6ccmhwrwrp9igg03575jg-nixos-15.09.git.5fd87e1M.drv --dry-run --option binary-caches-parallel-connections <N>'

gave the following mean elapsed times for these values of N:

N=10:  3.3541
N=20:  2.9320
N=25:  2.6690
N=30:  2.9417
N=50:  3.2021
N=100: 3.5718
N=150: 4.2079

Memory usage is also reduced (N=150 used 186 MB, N=25 only 68 MB).

Closes #708.

- rename options but leav old names as lower-priority aliases,
  also "-dirs" -> "-paths" to get closer to the meaning
- update docs to reflect the new names (old aliases are not documented),
  including a new file with release notes
- tests need an update after corresponding changes to nixpkgs
- __noChroot is left as it is (after discussion on the PR)

Passing "--option build-repeat <N>" will cause every build to be
repeated N times. If the build output differs between any round, the
build is rejected, and the output paths are not registered as
valid. This is primarily useful to verify build determinism. (We
already had a --check option to repeat a previously succeeded
build. However, with --check, non-deterministic builds are registered
in the DB. Preventing that is useful for Hydra to ensure that
non-deterministic builds don't end up getting published at all.)

This allows overriding the name component of the resulting Nix store
path, which is necessary if the base name of the URI contains
"illegal" characters.

This allows nix-prefetch-url to prefetch the output of fetchzip and
its wrappers (like fetchFromGitHub). For example:

  $ nix-prefetch-url --unpack https://github.com/NixOS/patchelf/archive/0.8.tar.gz

or from a Nix expression:

  $ nix-prefetch-url -A nix-repl.src

In the latter case, --unpack can be omitted because nix-repl.src is a
fetchFromGitHub derivation and thus has "outputHashMode" set to
"recursive".

See https://github.com/NixOS/nixos-homepage/commit/d4a88df9d6114f609da889db62ef0a3bf6e4ef27

For example,

  $ nix-prefetch-url -A hello.src

will prefetch the file specified by the fetchurl call in the attribute
‘hello.src’ from the Nix expression in the current directory. This
differs from ‘nix-build -A hello.src’ in that it doesn't verify the
hash.

You can also specify a path to the Nix expression:

  $ nix-prefetch-url ~/Dev/nixpkgs -A hello.src

List elements (typically used in ‘patches’ attributes) also work:

  $ nix-prefetch-url -A portmidi.patches.0

The Perl WWW::Curl bindings are required to build Nix.

Fixes https://github.com/NixOS/nixos-homepage/issues/46.

This can be used to implement functions like ‘imap’ (or for that
matter, ‘map’) without the quadratic complexity incurred by calling
‘++’ repeatedly.

This is a generalisation of replaceChars in Nixpkgs.

These are used thousands of times during NixOS evaluation, so it's
useful to speed them up.

And make exportPath() less spammy by default.