Create debian & rpm packages.

`debian:packageBin` and `rpm:packageBin` create `scala-$version.[deb|rpm]` in `target/`

- subdirs not requiring special treatment are rooted under `installTargetUnix.value`
- docs go to `installTargetUnixDocs.value`
- create symlinks under /usr/bin/ for all scripts in `installTargetUnix.value / "bin"`
- make scripts in `installTargetUnix.value / "bin"` executable, drop ".bat" files
- map man to /usr/share/man/
- map doc to `installTargetUnixDocs.value`
- map api to `installTargetUnixDocs.value / "api"`