Add a "prune" command

[vanrein]

Destroy all versions except for the one selected (either default_VERSION or through $VERSION_pkg) for any given package. This removes other /dl/pkg-^V, /src/pkg-^V, /build/pkg-^V and /tree/pkg-^V directories.

Consider support for a few variations:

• prune to erase other versions /.../pkg-^V
• purge to remove all versions /.../pkg-*
• clean to retain built /tree/pkg-*