move readmes/rcscripts generation into a separate target
'generate-readmes' that's always run, without a cookie.
this allows fine tweaks to that part without needing to rerun
fake every time.
Also places the automatic creation of PKGDIR where it belongs (in
update-plist) and have the new target error out implicitly if
PKGDIR is wrong.
slightly tweaked version compared to published tech@ patch, as it
turns out that scraping old files each time doesn't work,
in practice, as some readmes come from the actual distfile...
UnifiedSplitRaw
