Enh: consider whether to optionally prefix '>From ' lines when exporting/importing

The code now prefixes body text 'From ' lines with '>' when exporting to an mbox file.
This is necessary to ensure that the start of each message can be found correctly, and is known as the 'mboxo' format.
However this means that a line that starts '>From ' is indistinguishable from a line that starts with 'From ' in the generated file.

A work-round for this is to prefix all lines that start with 'From ' preceeded by 0 or more '>' chars.
This is known as the 'mboxrd' format.
This is (unfortunately) not the standard, so it may be best to make it optional. Likewise when importing messages from mbox files.

The mboxrd format is not currently supported by Python email

No files or tests are named. Locate the mbox export and import entry points, read how the current mboxo escaping works, and determine how an optional mboxrd mode should behave in both directions; done means the option is defined and its handling is covered by tests.

Written by the indexing model from the issue text.