New upstream version 2.6
Merge tag 'upstream/2.6'

Upstream version 2.6
debian/control: update dependencies
add debian/control.in
update debian/rules to handle control.in
debian/compat: increase to 9
debian/copyright: update years
debian/changelog: update date and version number
debian/control: autoupdate (overwritten by control.in)
debian/control.in: add build dependencies
debian/control.in: remove dot in short description to silence description-synopsis-might-not-be-phrased-properly
debian/control.in: add Filippo Rusconi to Uploaders to silence no-human-maintainers
debian/lintian-overrides: add extended-description-is-probably-too-short