sbbs

For messages that have no body, we'd report 0. But RFC 1939 says this value
is supposed to be "the exact size of the message in octets" <sigh>. This
would include header fields, so while we can't know the exact RFC822 size,
adding the msg's hdr.length to these values gives size > 0 for messages
with no body text and this enables the Apple iPhone Mail app to download
the message (fixing issue #822).

Part two of this fix is to provide a blank line of message text when there
is none. This changes the message displayed in the iPhone Mail app from:

"This message cannot be displayed because of the way it is formatted.
 Ask the sender to send it again using a different format or email program.

 text/plain"

to (the much nicer):
 "This message has no content".