This isn't used anywhere except in the configure script of the Perl
bindings. I've changed the latter to use the C++ API's Settings object
at runtime.

This means we don't have two (divergent) sets of option descriptions
anymore.

I gather this comes from the new unit tests.

Closes #3225.
Closes #3226.

This is no longer needed.

Previously, plain derivation paths in the string context (e.g. those
that arose from builtins.storePath on a drv file, not those that arose
from accessing .drvPath of a derivation) were treated somewhat like
derivaiton paths derived from .drvPath, except their dependencies
weren't recursively added to the input set. With this change, such
plain derivation paths are simply treated as paths and added to the
source inputs set accordingly, simplifying context handling code and
removing the inconsistency. If drvPath-like behavior is desired, the
.drv file can be imported and then .drvPath can be accessed.

This is a backwards-incompatibility, but storePath is never used on
drv files within nixpkgs and almost never used elsewhere.

This avoids sandbox annoyances.

All plugins in plugin-files will be dlopened, allowing them to
statically construct instances of the various Register* types Nix
supports.

It was getting too much like whac-a-mole listing all the retriable error
conditions, so we now retry by default and list the cases where retrying
is almost certainly hopeless.

Even with "build-use-sandbox = false", we now use sandboxing with a
permissive profile that allows everything except the creation of
setuid/setgid binaries.

Also, add rules to allow fixed-output derivations to access the
network.

These rules are sufficient to build stdenvDarwin without any
__sandboxProfile magic.

Issue #759.

Also, remove nix.conf from the sandbox since I don't really see a
legitimate reason for builders to access the Nix configuration.

Replaced by SSHStore.

Tests fail currently because the database is not given proper hashes in the VM

Rarely used, nix copy replaces it.

Refs #831

This was a dumb line-for-line rewrite, because nix build/nix run/etc.
will replace it.

This is primarily to subsume the functionality of the
copy-from-other-stores substituter. For example, in the NixOS
installer, we can now do (assuming we're in the target chroot, and the
Nix store of the installation CD is bind-mounted on /tmp/nix):

  $ nix-build ... --option substituters 'local?state=/tmp/nix/var&real=/tmp/nix/store'

However, unlike copy-from-other-stores, this also allows write access
to such a store. One application might be fetching substitutes for
/nix/store in a situation where the user doesn't have sufficient
privileges to create /nix, e.g.:

  $ NIX_REMOTE="local?state=/home/alice/nix/var&real=/home/alice/nix/store" nix-build ...