rework automatic build versioning

Only release or preview builds now receive the designated version
as their identifier. Others are now known as 'git-something', or,
for debs/rpms '0.0-something' (or similar).

A new jenkins-driver script is included that moves actually setting
the versions out of the jenkins configuration.
Extending this script to drive the complete build is left for the
future.
Merge branch 'f/versioning' of github.com:zeha/pdns