Use lutimes() if available to canonicalise the timestamp of symlinks

Also use utimes() instead of utime() if lutimes() is not available.
nix-build: Support the ‘-’ argument to build an expression from stdin
optimiseStore(): Use a content-addressed file store in /nix/store/.links

optimiseStore() now creates persistent, content-addressed hard links
in /nix/store/.links.  For instance, if it encounters a file P with
hash H, it will create a hard link

  P' = /nix/store/.link/<H>

to P if P' doesn't already exist; if P' exist, then P is replaced by a
hard link to P'.  This is better than the previous in-memory map,
because it had the tendency to unnecessarily replace hard links with a
hard link to whatever happened to be the first file with a given hash
it encountered.  It also allows on-the-fly, incremental optimisation.
Automatically optimise the Nix store when a new path is added

Auto-optimisation is enabled by default.  It can be turned off by
setting auto-optimise-store to false in nix.conf.
Garbage collect unused links in /nix/store/.links

Incremental optimisation requires creating links in /nix/store/.links
to all files in the store.  However, this means that if we delete a
store path, no files are actually deleted because links in
/nix/store/.links still exists.  So we need to check /nix/store/.links
for files with a link count of 1 and delete them.
Add a test for Nix store optimisation
Unlink the right file
Handle platforms that don't support linking to a symlink

E.g. Darwin doesn't allow this.
Disable tests temporarily
import: If the path is a valid .drv file, parse it and generate a derivation attrset.

The generated attrset has drvPath and outPath with the right string context, type 'derivation', outputName with
the right name, all with a list of outputs, and an attribute for each output.

I see three uses for this (though certainly there may be more):

* Using derivations generated by something besides nix-instantiate (e.g. guix)

* Allowing packages provided by channels to be used in nix expressions. If a channel installed a valid deriver
  for each package it provides into the store, then those could be imported and used as dependencies or installed
  in environment.systemPackages, for example.

* Enable hydra to be consistent in how it treats inputs that are outputs of another build. Right now, if an
  input is passed as an argument to the job, it is passed as a derivation, but if it is accessed via NIX_PATH
  (i.e. through the <> syntax), then it is a path that can be imported. This is problematic because the build
  being depended upon may have been built with non-obvious arguments passed to its jobset file. With this
  feature, hydra can just set the name of that input to the path to its drv file in NIX_PATH
Turn tests back on
prim_import: When importing .drvs, allocate the intermediate attrset on the heap just in case it escapes the stack frame.
Subscribe to the Nixpkgs rather than NixOS channel
nix-profile.sh: Don't set NIX_REMOTE on single user installations

Commit 6a214f3e06fa1c5f0a4d40e555f14d87691af297 reused the NixOS
environment initialisation for nix-profile.sh, but this is
inappropriate on systems that don't have multi-user support enabled.
Set permissions on temporary build directories to 0700

Fixes #39.
Document the --option flag

Pointed out by Daniel Santa Cruz on IRC.
Manual: Remove reference to non-existent -I option
Make ‘nix-store --optimise’ interruptible
Doh
removeUnusedLinks(): Print stats on disk space saved by hard linking