Call ldconfig in RPM and Debian packages

The RPM and Debian package installers don't automatically call `ldconfig` for you if your package contains shared libraries.  So when building packages for those systems, we need to include an explicit call to `ldconfig` in the post-install and post-remove scripts.  Ideally we'd check whether the package contains any shared libraries, and only add in the call if necessary, but we don't currently have that level of introspection available.  So we call it for all buzzy-built packages.

Note that pacman **does** call `ldconfig` for us, so we don't need to add any explicit calls on that platform.
No this is run when the binary package file is installed — Buzzy isn't necessarily present at that point.  Also this is run as root, so `$HOME` wouldn't refer to the user who created the package.