Handle messages without the closing multipart boundary

[mjl-]

The message sent by @gdunstone in #298 was missing the MIME "boundary" that properly ends the multipart message. Mox treats those messages as having an invalid MIME structure, and treats it whole message as essentially an application/octet-stream blob. Other mail clients are more lenient, pretend the message does have the missing closing boundary, and if the structure then makes sense, will show the message "as intended". Of course, it can't be known if the message was intended to end at the missing boundary or that is is really truncated (perhaps we can have heuristics for that).

We should probably also have parser that is lenient to missing the final closing boundary, enabled by default, but not when in pedantic mode.

[gdunstone]

I think the aim is to be forgiving when receiving, but strict when sending.

Is there a "pedantic mode" that can be enabled?