lib: resurrect support for single-message mbox files

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.