distro: differentiate upstream packages

Users reported conflicts between upstream and downstream packages. In order to differentiate, I propose to insert cznic. prefix into package release which will lead to a preference of upstream packages over downstream ones when same versions are available:

1.2.3-1 < 1.2.3-cznic.1
1.2.3-999999999 < 1.2.3-cznic.1

Furthermore it's now possible to tell whether upstream or downstream packages are in use just by inspecting installed packages' releases which should make debugging easier. For example:

dpkg -l | grep cznic

Equivalent change has been merged in Knot Resolver: knot-resolver@58f48b2b