Clean up buildscripts

- move scripts from buildscripts/ according to the new ROADMAP, create
three directories for this purpose: scripts/aux/, scripts/build/ and
python/aux;

- make permissions and interpreter specs clearer: all scripts in
scripts/build are non-executable and use an interpreter determined in
config.make, all scripts in scripts/aux use hard-coded interpreters;

- always use built avatars of scripts in scripts/build, as a
consequence remove interpreter invocations which are no longer needed;

- keep old scripts in buildscripts/ which are junked in the very next
commits.