lei: support reading MH for convert+import+index
    
    The MH format is widely-supported and used by various MUAs such
    as mutt and sylpheed, and a MH-like format is used by mlmmj for
    archives, as well.  Locking implementations for writes are
    inconsistent, so this commit doesn't support writes, yet.
    
    inotify|EVFILT_VNODE watches aren't supported, yet, but that'll
    have to come since MH allows packing unused integers and
    renaming files.