send: Add --force-cover-letter to send separate cover for single patch patch series

By default, b4 mixes the cover letter content into the patch body
(under the --- cut line) when sending a single-patch series. Add
--force-cover-letter option to allow sending a separate cover letter
message (as 0/1) even for single-patch series.

This is useful when developers want to send a long cover letter with
extra information about an issue, and a commit. Having the cover-letter
"mixed-in" makes it hard to follow.

Example:
 * https://lore.kernel.org/all/20260127-ipmi-v1-0-ba5cc90f516f@debian.org/

Signed-off-by: Breno Leitao <leitao@debian.org>
Tested-by: Luca Ceresoli <luca.ceresoli@bootlin.com>
Link: https://patch.msgid.link/20260127-force-cover-letter-v1-1-1ba8d7f9a64f@debian.org
Signed-off-by: Konstantin Ryabitsev <konstantin@linuxfoundation.org>