virtualbox: Disable depmod only where necessary.

Traversing the full source tree is unneccessary, because the calls are
only done within make files. Hence we only substitute make files now.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>
virtualbox: Enable hardening by default.

VirtualBox with hardening support requires the main binaries to be
setuid root. Using VBOX_WITH_RUNPATH, we ensure that the RPATHs are
pointing to the libexec directory and we also need to unset
VBOX_WITH_ORIGIN to make sure that the build system is actually setting
those RPATHs.

The hardened.patch implements two things:

 * Set the binary directory to the setuid-wrappers dir so that
   VboxSVC calls them instead of the binaries from the store path. The
   reason behind this is because nothing in the Nix store can have the
   setuid flag.
 * Excempt /nix/store from the group permission check, because while it
   is group-writeable indeed it also has the sticky bit set (and also
   the whole store is mounted read-only on most NixOS systems), so we're
   checking on that as well.

Right now, the hardened.patch uses /nix/store and /var/setuid-wrappers
directly, so someone would ever want to change those on a NixOS system,
please provide a patch to set those paths on build time. However, for
simplicity, it's best to do it when we _really_ need it.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>