mq: smarter handling of plain headers 6333412245ec and 5ccced6eab0b fixed issue4453 with a simple insertplainheader function that fixed the regression but didn't make the implementation more stable. Now we introduce plain header handling similar to how we handle hg patches. The whole header is scanned for fields to update while determining the best position for inserting the field if it is missing. It also makes sure there is an empty line between headers and body.