vfork() is just too weird.  For instance, in this build:

  http://hydra.nixos.org/build/3330487

the value fromHook.writeSide becomes corrupted in the parent, even
though the child only reads from it.  At -O0 the problem goes away.
Probably the child is overriding some spilled temporary variable.

If I get bored I may implement using posix_spawn() instead.

Slightly scared of using std::cerr in a vforked process...

Hopefully this reduces the chance of hitting ‘unable to fork: Cannot
allocate memory’ errors.  vfork() is used for everything except
starting builders.

They are unnecessary because we set the close-on-exec flag.

http://hydra.nixos.org/build/3313227

Since the called function can return its argument attribute set
(e.g. "a"), the latter should not be allocated on the stack.

Reported by Shea.

Slight optimisation.

We have close-on-exec on all FDs now, and there is no security risk in
passing open FDs to substituters anyway.

Binary caches can now specify a priority in their nix-cache-info file.
The binary cache substituter checks caches in order of priority.  This
is to ensure that fast, static caches like nixos.org/binary-cache are
processed before slow, dynamic caches like hydra.nixos.org.

This allows disabling the use of binary caches, e.g.

  $ nix-build ... --option use-binary-caches false

Note that

  $ nix-build ... --option binary-caches ''

does not disable all binary caches, since the caches defined by
channels will still be used.

It's a mystery why this error is not triggered in the build farm
(e.g. http://hydra.nixos.org/build/3265602).  Ah well.

Fixes #57.

If ‘--link’ is given, nix-push will create hard links to the NAR files
in the store, rather than copying them.  This is faster and requires
less disk space.  However, it doesn't work if the store is on a
different file system.

This prevents unnecessary and slow rebuilds of NARs that already exist
in the binary cache.

Reported by Shea.

Ignoring assertion failures makes some sense for nix-env -qa, but not
for nix-instantiate/nix-build or hydra-eval-jobs.

This is useful for hydra-eval-jobs.

http://hydra.nixos.org/build/3124130

http://hydra.nixos.org/build/3123177

AFAIK nobody uses this, setuid binaries are evil, and there is no good
reason why people can't just run the daemon.