Enh: add support for mboxrd format

[sebbASF]

Body lines in mbox files must not start with the characters 'From ' as the string identifies the start of the next message.

Unfortunately there is no single standard for how such lines are handled.

The mboxo format changes 'From ' to '>From '; this is ambiguous when attempting to regenerate the original email.

The mboxrd format additionally changes '>From ' to '>>From ' etc, which is reversible.

Apache HTTPD mod_mbox currently uses mboxo (as far as I am aware).
The ASF mbox-vm archiver uses mboxrd.

PonyMail currently uses mboxo for generating mbox downloadfiles and defaults to reading mbox files as mboxo.

Whilst this is self-consistent, it is not ideal.
The code should allow for mboxrd support.