mq: when setting message in plain mode, separate it from header (issue4453)

Fix inconsistent handling of plain header separation in mq patcheader - and
contrary to c87f2a5a6e49, do it in the direction of having an empty line
between header and description. Plain patches are like mails and should thus
have an empty line between headers and body in compliance with RFC 822 3.1.

--HG--
branch : stable
mq: introduce insertplainheader - same naive implementation as before

--HG--
branch : stable
mq: when adding headers in plain mode, separate them from message (issue4453)

c87f2a5a6e49 did a clean-up in one direction ... but we want it in the other
direction.

--HG--
branch : stable