We now have the VCL_SUB type in varnish-cache, which exists mainly to
make this vmod obsolete.
Yet we need to update vmod_dispatch to enable a smooth transition to the
replacement (likely vmod_re2 with VCL_SUB support added), so we keep the
existing semantics and just change the call interface.
*NOTE* We need to do here what VMODs are _explicitly_ not supposed to do
and make a VSUB_STATIC call. This is necessary, because VSUB_DYNAMIC
calls are only enabled through VSUB references, and to keep the vmod
interface, we need to keep the .add() argument a strig.