Pointed out by @cstrahan, thanks!

Conflicts:
	src/libexpr/eval.cc

The value pointers of lists with 1 or 2 elements are now stored in the
list value itself. In particular, this makes the "concatMap (x: if
cond then [(f x)] else [])" idiom cheaper.

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

Such as whether Nix is built with signed binary cache support, and the
location of the configuration file.

Turns out getgrouplist() is not POSIX.

http://hydra.nixos.org/build/23881243

This ensures that 1) the derivation doesn't change when Nix changes;
2) the derivation closure doesn't contain Nix and its dependencies; 3)
we don't have to rely on ugly chroot hacks.

In particular, hydra-queue-runner can now distinguish between remote
build / substitution / already-valid. For instance, if a path already
existed on the remote side, we don't want to store a log file.

And make exportPath() less spammy by default.

This is mostly useful for hydra-queue-runner.

Previously, to build a derivation remotely, we had to copy the entire
closure of the .drv file to the remote machine, even though we only
need the top-level derivation. This is very wasteful: the closure can
contain thousands of store paths, and in some Hydra use cases, include
source paths that are very large (e.g. Git/Mercurial checkouts).

So now there is a new operation, StoreAPI::buildDerivation(), that
performs a build from an in-memory representation of a derivation
(BasicDerivation) rather than from a on-disk .drv file. The only files
that need to be in the Nix store are the sources of the derivation
(drv.inputSrcs), and the needed output paths of the dependencies (as
described by drv.inputDrvs). "nix-store --serve" exposes this
interface.

Note that this is a privileged operation, because you can construct a
derivation that builds any store path whatsoever. Fixing this will
require changing the hashing scheme (i.e., the output paths should be
computed from the other fields in BasicDerivation, allowing them to be
verified without access to other derivations). However, this would be
quite nice because it would allow .drv-free building (e.g. "nix-env
-i" wouldn't have to write any .drv files to disk).

Fixes #173.

Fixes #572.

Don't barf trying to read a link that just got deleted.

Fixes #575.

This modification moves Attr and Bindings structures into their own header
file which is dedicated to the attribute set representation. The goal of to
isolate pieces of code which are related to the attribute set
representation. Thus future modifications of the attribute set
representation will only have to modify these files, and not every other
file across the evaluator.

The following patch is an attempt to address this bug (see
<http://bugs.gnu.org/18994>) by preserving the supplementary groups of
build users in the build environment.

In practice, I would expect that supplementary groups would contain only
one or two groups: the build users group, and possibly the “kvm” group.

[Changed &at(0) to data() and removed tabs - Eelco]

This reverts commit 76f985b9. We
shouldn't mess with $MANPATH, because on some "man" implementations
(like NixOS'), the default value on $MANPATH is derived from $PATH. So
if you set $MANPATH, you lose the default locations.

Issue #564.

Issue #564.

This didn't work (despite claims in the manual), because the colon in
"http://" was parsed as a element separator. So handle "://"
specially.

Fixes #558.

This is useful for the new hydra-queue-runner.

Fixes #548.