[AC21.5] Find a relative package root at runtime

Here is the latest (final, I hope) version of my 21.5 package-finding patch. This patch will enable me to cons up a 21.5 native setup kit that will be compatible with the 21.4 kit. Changes since last version: 1. Per Michael's comments, runtime searching for a package root using xemacs-packages is only for win32 configuration (I took out the check for cygwin in setup-paths.el). 2. I ripped out all the mechanism in xemacs.mak to set PATH_PACKAGEPATH. Now only PACKAGE_PREFIX is recognized from config.inc and that gets directly translated into PATH_LATE_PACKAGE_DIRECTORIES. Stephen, I have not added an additional packages layer preceding {mule,site,xemacs}-packages. I think consensus ran against that additional layer. I'd like to apply this patch within a couple of days, so please try this out and send along your comments.