This prevents unnecessary and slow rebuilds of NARs that already exist
in the binary cache.

Put all Nix configuration flags in a Settings object.

Previously substituters could read nix.conf themselves, but this
didn't take --option flags into account.

To implement binary caches efficiently, Hydra needs to be able to map
the hash part of a store path (e.g. "gbg...zr7") to the full store
path (e.g. "/nix/store/gbg...kzr7-subversion-1.7.5").  (The binary
cache mechanism uses hash parts as a key for looking up store paths to
ensure privacy.)  However, doing a search in the Nix store for
/nix/store/<hash>* is expensive since it requires reading the entire
directory.  queryPathFromHashPart() prevents this by doing a cheap
database lookup.

Cherry-picked from the no-manifests branch.

XZ compresses significantly better than bzip2.  Here are the
compression ratios and execution times (using 4 cores in parallel) on
my /var/run/current-system (3.1 GiB):

  bzip2: total compressed size 849.56 MiB, 30.8% [2m08]
  xz -6: total compressed size 641.84 MiB, 23.4% [6m53]
  xz -7: total compressed size 621.82 MiB, 22.6% [7m19]
  xz -8: total compressed size 599.33 MiB, 21.8% [7m18]
  xz -9: total compressed size 588.18 MiB, 21.4% [7m40]

Note that compression takes much longer.  More importantly, however,
decompression is much faster:

  bzip2: 1m47.274s
  xz -6: 0m55.446s
  xz -7: 0m54.119s
  xz -8: 0m52.388s
  xz -9: 0m51.842s

The only downside to using -9 is that decompression takes a fair
amount (~65 MB) of memory.

Since the Perl bindings require shared libraries, this is required on
platforms such as Cygwin where we do a static build.

Perl on some 32-bit systems needs -D_FILE_OFFSET_BITS=64.  See also commit
02f1363e.

This fixes the Darwin build (http://hydra.nixos.org/build/2517380).
Hopefully it doesn't break other builds.

nix-copy-closure: Move the progressViewer directly adjacent to the ssh call so that network progress is what's measured

This command builds or fetches all dependencies of the given
derivation, then starts a shell with the environment variables from
the derivation.  This shell also sources $stdenv/setup to initialise
the environment further.

The current directory is not changed.  Thus this is a convenient way
to reproduce a build environment in an existing working tree.

Existing environment variables are left untouched (unless the
derivation overrides them).  As a special hack, the original value of
$PATH is appended to the $PATH produced by $stdenv/setup.

Example session:

$ nix-build --run-env '<nixpkgs>' -A xterm
(the dependencies of xterm are built/fetched...)
$ tar xf $src
$ ./configure
$ make
$ emacs
(... hack source ...)
$ make
$ ./xterm

We're already printing progress on stderr, so printing them on stdout
afterwards is kind of useless.

  Without these, Nix fails on 32-bit Linux with Perl 5.14, with a
  rather unhelpful error message:

    Not a CODE reference at /nix/store/n6kpbacn6nn7i3i735v8j3di8aqyl07v-perl-5.14.2/lib/perl5/5.14.2/i686-linux-thread-multi/DynaLoader.pm

  This is likely because the lack of -D_FILE_OFFSET_BITS=64 causes
  various Perl structures to not match what the Perl interpreter
  expects.

  Mac OS X instead of .dylib, so don't do that.

  scripts.
* Include the version and architecture in the -I flag so that there is
  at least a chance that a Nix binary built for one Perl version will
  run on another version.

* Refactor the nix-channel unpacker a bit.