As a side effect, this ensures that signatures are propagated when
copying paths between stores.

Also refactored import/export to make use of this.

Workaround to support ruby as an interpreter

fix "nix-build" examples

SystemV init file for BSD systems, old Linux distributions (RHEL 6) a…

nix-mode: set syntax and font properties for multiline strings

add Ubuntu 16.03 .deb builds

E.g.

  $ nix-build -I nixpkgs=git://github.com/NixOS/nixpkgs '<nixpkgs>' -A hello

This is not extremely useful yet because you can't specify a
branch/revision.

The function builtins.fetchgit fetches Git repositories at evaluation
time, similar to builtins.fetchTarball. (Perhaps the name should be
changed, being confusing with respect to Nixpkgs's fetchgit function,
with works at build time.)

Example:

  (import (builtins.fetchgit git://github.com/NixOS/nixpkgs) {}).hello

or

  (import (builtins.fetchgit {
    url = git://github.com/NixOS/nixpkgs-channels;
    rev = "nixos-16.03";
  }) {}).hello

Note that the result does not contain a .git directory.

Other stores don't do this either. It's up to the caller to check
signatures.

This is to allow store-specific configuration,
e.g. s3://my-cache?compression=bzip2&secret-key=/path/to/key.

Substitution is now simply a Store -> Store copy operation, most
typically from BinaryCacheStore to LocalStore.

The existing "nix-build" examples were failing:

  error: cannot auto-call a function that has an argument without a default value (‘system’)

Thanks to @groxxda on irc for pointing out the fix!

Updated to completely remove unneeded path argument, suggested by @joachifm

Updated to remove other occurences of `all-packages.nix` from files as
suggested by @domenkozar

If --no-build-output is given (which will become the default for the
"nix" command at least), show the last 10 lines of the build output if
the build fails.

This was added to support Hydra, but Hydra no longer uses it.

This also gets rid of --log-type, since the nested log type isn't
useful in a multi-threaded situation, and nobody cares about the
"pretty" log type.