Pass the architecture to the configure script

Some software may have architecture-dependent configuration, so the
configuration script needs to know which architecture is to be installed.
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
Cleanups to fpc-merged-postinst

* Create the directories if they don't exist
* Consistently use MAN_DIR, rather than occasionally MANDIR
Make more cross-architecture

* Actually support amd64, armel, armhc, and i386.
* Stop claiming to support anything else.