Done slightly differently from https://github.com/NixOS/nix/pull/1093.

This caused the gc test to fail on Darwin.

.gitignore: catch up with perl -> c conversion

This was broken since ff0c0b64. Since
I can't figure out how to mount a devpts instance in the sandbox,
let's just bind-mount the host devpts.

http://hydra.nixos.org/build/42025230

Commit 86e8c67e broke it, because
CURL_* are not actually #defines.

This prevents collisions with the "native" OpenSSL, in particular on
OS X.

Fixes #921.

This allows the binary cache substituter to pipeline requests.

override rx directory permissions in deletePath()

Fixes #1069.

We can now write

  throw Error("file '%s' not found", path);

instead of

  throw Error(format("file '%s' not found") % path);

and similarly

  printError("file '%s' not found", path);

instead of

  printMsg(lvlError, format("file '%s' not found") % path);

Document the { __toString } interface

Add a new option to disable documentation generation at configure time

We were passing "p=$PATH" rather than "p=$PATH;", resulting in some
invalid shell code.

Also, construct a separate environment for the child rather than
overwriting the parent's.

Otherwise the shell and its children will be bound to one CPU core...

The fact that queryPathInfo() is synchronous meant that we needed a
thread for every concurrent binary cache lookup, even though they end
up being handled by the same download thread. Requiring hundreds of
threads is not a good idea. So now there is an asynchronous version of
queryPathInfo() that takes a callback function to process the
result. Similarly, enqueueDownload() now takes a callback rather than
returning a future.

Thus, a command like

  nix path-info --store https://cache.nixos.org/ -r /nix/store/slljrzwmpygy1daay14kjszsr9xix063-nixos-16.09beta231.dccf8c5

that returns 4941 paths now takes 1.87s using only 2 threads (the main
thread and the downloader thread). (This is with a prewarmed
CloudFront.)