"cp -r" doesn't copy symlinks properly on Darwin, but "cp -R" does.

Fixes #215.

Fixes #68.
Fixes #117.

We were 1) using CURLOPT_TIMEOUT instead of CURLOPT_CONNECTTIMEOUT; 2)
not passing it to the curl child process.

Issue #93.

Fixes #112.

Fixes #121.  Note that we don't warn about missing $NIX_PATH entries
because it's intended that some may be missing (cf. the default
$NIX_PATH on NixOS, which includes paths like /etc/nixos/nixpkgs for
backward compatibility).

Signed-off-by: Shea Levy <shea@shealevy.com>

use USER environmental variable if getting user id by getpwuid is failed in perl scripts: download-from-binary-cache.pl and nix-channel

It defaults to false and can be overridden by RemoteStore.

Untested currently, just quickly put this together

This allows you to easily set up a build environment containing the
specified packages from Nixpkgs.  For example:

  $ nix-shell -p sqlite xorg.libX11 hello

will start a shell in which the given packages are present.

--dry-run is more consistent with nix-env and nix-store.

This is basically a shortcut for ‘echo 'expr...' | nix-instantiate -’.
Also supported by nix-build and nix-shell.

The flag ‘--check’ to ‘nix-store -r’ or ‘nix-build’ will cause Nix to
redo the build of a derivation whose output paths are already valid.
If the new output differs from the original output, an error is
printed.  This makes it easier to test if a build is deterministic.
(Obviously this cannot catch all sources of non-determinism, but it
catches the most common one, namely the current time.)

For example:

  $ nix-build '<nixpkgs>' -A patchelf
  ...
  $ nix-build '<nixpkgs>' -A patchelf --check
  error: derivation `/nix/store/1ipvxsdnbhl1rw6siz6x92s7sc8nwkkb-patchelf-0.6' may not be deterministic: hash mismatch in output `/nix/store/4pc1dmw5xkwmc6q3gdc9i5nbjl4dkjpp-patchelf-0.6.drv'

The --check build fails if not all outputs are valid.  Thus the first
call to nix-build is necessary to ensure that all outputs are valid.

The current outputs are left untouched: the new outputs are either put
in a chroot or diverted to a different location in the store using
hash rewriting.

Its linker is too old to understand --no-copy-dt-needed-entries.

http://hydra.nixos.org/build/9113883

Mac OS X doesn't allow renaming a read-only directory.

http://hydra.nixos.org/build/9113895

In particular, do replacing of valid paths during repair later.  This
prevents us from replacing a valid path after the build fails.

E.g. "--max-freed 10G" means "free ten gigabytes".

This will allow Hydra to detect that a build should not be marked as
"permanently failed", allowing it to be retried later.

Since normal builds don't execute shellHook, this allows nix-shell
specific customisation.  Suggested by Domen.