eml: each_part: single part $idx is 1

eml: each_part: single part $idx is 1

Instead of counts starting at 0, we start the single-part
message at 1 like we do with subparts of a multipart message.
This will make it easier to map offsets for "BODY[$SECTION]"
when using IMAP FETCH, since $SECTION must contain non-zero
numbers according to RFC 3501.

This doesn't make any difference for WWW URLs, since single part
messages cannot have downloadable attachments.