Add "rpmextract" script and use it

In Arch Linux there is a tiny tool called "rpmextract" which basically just calls rpm2cpio and cpio -d. It, however, saves a lot of time when working with rpm archives (standalone and while packaging smth) and helps a person who doesn't know anything about rpm to just "extract" it (it helped me, at least ^_^).
I've reimplemented this little tool (it lives as a script in Arch Linux packages, and so tiny there's no sense in using it from there) and replaced most uses of rpm+cpio with it. There are also some uses in build-support, but I omitted them for now.