Drop use of "stable", "testing" and "unstable" monikers

With the current state of the Devuan releases it is unclear what "stable" and "testing" refer to. The most recent Devuan release is jessie but using stable in one's APT sources will result in getting packages from Debian's current stable which is stretch. That is currently the base for Devuan's ascii.

The idea is to completely refrain from using the non-changing release identifiers, i.e. stable, testing and unstable. Instead only the code names will be used.