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.
This one is incorrect. Windows has to look inside ``/var/cfengine`` specifically, because it's own workdir is ``C:\Program Files\Cfengine``. Ideally this should be solved using cf-serverd shortcuts, but for now we need this discrepancy.