Respect DESTDIR if set as unpack destination

Some environments will want to unpack to a directory other than /, for
the purposes of repacking the content into another (non-.deb) format.
This is indicated by setting DESTDIR to the desired location.
Add README

Add a short README file to give context.
Rename to debinject
Configuration for fpc

    * debinject.conf referencing fpc
    * fpc-merged-postinst including all the relevant postinst bits