Save VPI_count() info to struct wrk_vpi

So we have it available for Panics in a follow-up commit

bin/varnishd/cache/cache_vpi.c

@@ -67,6 +67,7 @@ VPI_count(VRT_CTX, unsigned u)
 	CHECK_OBJ_NOTNULL(ctx->vcl, VCL_MAGIC);
 	CHECK_OBJ_NOTNULL(ctx->vcl->conf, VCL_CONF_MAGIC);
 	assert(u < ctx->vcl->conf->nref);
+	ctx->vpi->ref = u;
 	if (ctx->vsl != NULL)
 		VSLb(ctx->vsl, SLT_VCL_trace, "%s %u %u.%u.%u",
 		    ctx->vcl->loaded_name, u, ctx->vcl->conf->ref[u].source,

include/vcc_interface.h

@@ -58,6 +58,7 @@ struct wrk_vpi {
 	unsigned	magic;
 #define WRK_VPI_MAGIC	0xaa3d3df3
 	unsigned	handling;
+	unsigned	ref;	// index into (struct vpi_ref)[]
 };