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
INSTALLATION_DIRS, as well as all occurrences of ${PREFIX}/man with
${PREFIX}/${PKGMANDIR}.
Fixes PR 35265, although I did not use the patch provided therein.
