Update the Zsh pkgtools.

Update the Zsh pkgtools.

Added a function to find reverse dependencies and fixed a couple of minor
niggling things.

	* etc/pkgusr/zsh/zsh-pkgtools (pkgwant): New function that lists
	packages that depend on a certain pkg.

	* etc/pkgusr/zsh/zsh-pkgtools (H-pkg): Mention pkgwant().

	* etc/pkgusr/zsh/zsh-pkgtools (xtar,vtar): Fix bug where xz files
	weren't being detected.

	* etc/pkgusr/zsh/zsh-pkgtools (pkg_ldconfig): Prevent spurious
	output.

	* etc/pkgusr/zsh/_zsh-pkgtools: Tab-complete users for pkgwant.

Signed-off-by: Steve Youngs <steve@sxemacs.org>