test: use --quick when starting emacs.

At least in emacs24, this removes the "site-lisp" directories from the
load path in addition to enforcing --no-site-lisp --no-init-file.

This works around a slightly mysterious bug on Debian that causes
test-lib.el not to load when there is cl-lib.el(c) in some site-lisp
directory.  It should be harmless in general since we really don't
want to load any files from addon packages to emacs.
lib: resurrect support for single-message mbox files

This is effectively a revert of

commit 6812136bf576d894591606d9e10096719054d1f9
Author: Jani Nikula <jani@nikula.org>
Date:   Mon Mar 31 00:21:48 2014 +0300

    lib: drop support for single-message mbox files

The intention was to drop support for indexing new single-message mbox
files (and whether that was a good idea in the first place is
arguable). However this inadvertently broke support for reading
headers from previously indexed single-message mbox files, which is
far worse.

Distinguishing between the two cases would require more code than
simply bringing back support for single-message mbox files.