builddeb: rename --release option to --codename...
builddeb: rename --release option to --codename
Debian and Ubuntu releases have both codenames and traditional version numbers.
An entire "branch" of releases is referred to by its codename, and version
numbers (e.g. 8.2, 14.04.3) are used to address individual releases.
Since we use codenames for building .deb packages, let's call the option and
the variable appropriately.