20090508:
	Added -c flag, clean cache: delete all downloaded packages from
	/var/db/pkg_dry/cache

20090507:
	A database rebuild is needed, along with a regeneration of
	the ddb_create.h header in order to support multi-repository.

	Record your keepables packages :

	$ pkg_dry -p|cut -d' ' -f1 > pkg_dry.keep

	Remove the database

	# rm -f /var/db/pkg_dry/pkg_dry.db

	Set the PKG_REPOS variable to your repository(ies) :

	# PKG_REPOS=ftp://ftp.fr.netbsd.org/pub/pkgsrc/packages/NetBSD/i386/5.0/All
	# export PKG_REPOS

	Rebuild pkg_dry

	$ rm -f ddb_create.h
	$ make

	Regenerate the database

	# pkg_dry -u

	Mark your keepables packages

	# for pkg in `cat pkg_dry.keep`; do pkg_dry -k $pkg; done