d/watch: add signature verification

Lets add signature verification.

Neutron Soutmun reports that starting with devscripts 2.14.0, the signature
is expected to live in debian/upstream/

Signed-off-by: Arturo Borrero Gonzalez <aborrero@cica.es>
d/control: lower priority

Do lower the priority to 'extra'.

Signed-off-by: Arturo Borrero Gonzalez <aborrero@cica.es>