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 ...

Manifests have been superseded by binary caches for years. This also
gets rid of nix-pull, nix-generate-patches and bsdiff/bspatch.

Grmbl...

Autoreconf doesn't install these if you're not using Automake.

AFAIK, nobody uses it, it's not maintained, and it has no tests.

I.e. do what git does.  I'm too lazy to keep the builtin help text up
to date :-)

Also add ‘--help’ to various commands that lacked it
(e.g. nix-collect-garbage).