When searching for inactive maintainers it's useful to filter
out mailing list addresses. Such "maintainers" will obviously
never feature in a "From:" line of an email or a review tag.

Since "L:" entries only provide the address of a mailing list
without a fancy name extend this pattern to "M:" entries.

Alternatively we could reserve M: entries for humans only
and move the fake "maintainers" to L:. While I'd personally
prefer to reserve M: for humans only, I'm not 100% that's
a great choice either, given most L: entries are in fact
open mailing lists with public archives.

Link: https://lore.kernel.org/r/20201219185538.750076-1-kuba@kernel.org


Signed-off-by: Jakub Kicinski <kuba@kernel.org>