This version replaces (no longer appends) the old checksums instead
of using those specified by INTEGRITY_CHECK in makepkg.conf. If there
are multiple checksums in the PKGBUILD, it will replace the first one
and move the new ones to it's place.
It also uses a file called "upgpkg" as a scriptlet file (similar to
pacman's install files) It can be generated by running upgpkg -g. The
functions used could also be defined in the PKGBUILD, but I think that's
not really what one should do.