Shared directories can now be created independently by the pacakges
needing them and will be removed automatically by pkg_delete when empty.
Packages needing empty directories can use the @pkgdir command in PLIST.
Discussed and ok'd in thread starting at
http://mail-index.netbsd.org/tech-pkg/2009/06/30/msg003546.html
- assume that Python 2.4 and 2.5 are compatible and allow checking for
fallout.
- remove PYTHON_VERSIONS_COMPATIBLE that are obsoleted by the 2.3+
default. Modify the others to deal with the removals.