Generalise tar extraction technique

    Modern tar appears to be able to correctly detect the compression
in use with only xf, rather than xzf, xjf, xJf, etc.  Use this so that
we don't have to understand which compression technique is in use.
Configuration for fpc

    * debinject.conf referencing fpc
    * fpc-merged-postinst including all the relevant postinst bits
Update to 2.6.4+defsg-2

* Update configuration to point to new version
* Update postinst to reference new version
* Add $DESTDIR in postinst