Conflicts:
	doc/manual/release-notes.xml
	doc/manual/writing-nix-expressions.xml

Apparently, turning on utf8 encoding on stderr changes its flushing
behaviour, causing sendReply to not send anything.

http://hydra.nixos.org/build/13944384

Fixes #333.

For the "stdenv accidentally referring to bootstrap-tools", it seems
easier to specify the path that we don't want to depend on, e.g.

  disallowedRequisites = [ bootstrapTools ];

I don't think it's a good idea to use allowedRequisites for stdenv, so
don't mention it here.

So all these years I was totally deluded about the meaning of "set
-e". You might think that it causes statements like "false && true" or
"! true" to fail, but it doesn't...

It turns out that using clone() to start a child process is unsafe in
a multithreaded program. It can cause the initialisation of a build
child process to hang in setgroups(), as seen several times in the
build farm:

The reason is that Glibc thinks that the other threads of the parent
exist in the child, so in setxid_mark_thread() it tries to get a futex
that has been acquired by another thread just before the clone(). With
fork(), Glibc runs pthread_atfork() handlers that take care of this
(in particular, __reclaim_stacks()). But clone() doesn't do that.

Fortunately, we can use fork()+unshare() instead of clone() to set up
private namespaces.

See also https://www.mail-archive.com/lxc-devel@lists.linuxcontainers.org/msg03434.html.

This was triggered by 47e18584, which
turned globals.state into a pointer.

Borrowed from systemd.