Log Message:
Remove USE_DIRS from pkgsrc.

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