checkpatch: detect unhandled placeholders in cover letters

checkpatch: detect unhandled placeholders in cover letters

Add a new check PLACEHOLDER_USE to detect unhandled placeholders.  This
prevents sending patch series with incomplete patches (mostly in cover
letters) containing auto generated subject or blurb lines.

These placeholders can be seen on mailing lists.  With this change,
checkpatch will emit an error when such text is found.

Link: https://lkml.kernel.org/r/20250917173725.22547-2-work@onurozkan.dev
Signed-off-by: Onur Özkan <work@onurozkan.dev>
Acked-by: Joe Perches <joe@perches.com>
Cc: Andy Whitcroft <apw@canonical.com>
Cc: Dwaipayan Ray <dwaipayanray1@gmail.com>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Lukas Bulwahn <lukas.bulwahn@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>