dm: update maintainers

Add Mike Snitzer as an additional device-mapper maintainer.
Recognise uapi dm-*.h include files as device-mapper files.

Signed-off-by: Alasdair G Kergon <agk@redhat.com>
Cc: Yann Droneaud <ydroneaud@opteya.com>
Cc: Mike Snitzer <snitzer@redhat.com>