Make workdir relocatable.

This commit changes most occurances of `/var/cfengine` with `$(sys.workdir)` and `/var/cfengine/bin` with `$(sys.bindir)`. This helps make the default policy set more useful when users compile cfengine with alternate workdir paths.
Care must be taken here as well. Windows needs ``/var/cfengine`` specifically. Since this is for automatic upgrades of Enterprise, I propose that this one be left alone, since Enterprise packages will always be built with ``/var/cfengine`` as workdir.