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>