http://hydra.nixos.org/build/14344391

Emacs 24.1 introduced the notion of "basic major modes" and among these
is prog-mode, see section "23.2.5 Basic Major Modes" in the Emacs
manual. The prog-mode basic major mode is recommended as a base for
derived major modes that are intended for editing source code.

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.