Refactor mvn / mvnw scripts to share common core

Before merging the maven wrapper, it was clear that the scripts of mvn evolved, but were never updated for mvnw.
When bringing the scripts in sync, the goal was to keep the mvn scripts close to the original.
However, this caused quite fragmented scripts. This is pretty hard to read and to maintain.

It should be possible to reorder the scripts. Ideally there is 1 base which can be merged with the mvn and mvnw specific script.