Apparently, turning on utf8 encoding on stderr changes its flushing
behaviour, causing sendReply to not send anything.

http://hydra.nixos.org/build/13944384

This prevents collisions with other users.

Fixes #262.

These are too difficult to implement via nix-store --serve.

‘--show-progress’ could be re-implemented fairly easily via a
sink/source wrapper class.

This makes things more efficient (we don't need to use an SSH master
connection, and we only start a single remote process) and gets rid of
locking issues (the remote nix-store process will keep inputs and
outputs locked as long as they're needed).

It also makes it more or less secure to connect directly to the root
account on the build machine, using a forced command
(e.g. ‘command="nix-store --serve --write"’). This bypasses the Nix
daemon and is therefore more efficient.

Also, don't call nix-store to import the output paths.

This means we no longer need an SSH master connection, since we only
execute a single command on the remote host.

If derivation declares multiple outputs and first (default) output
if not "out", then "nix-instantiate" calls return path with output
names appended after "!". Than suffix must be stripped before
ant path checks are done.

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

Fixes #215.

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

Issue #93.

Fixes #112.

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

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.

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