Add VRT_AllocStrandsWS() to allocate strands on a workspace

So far, we have focused on transitioning vmod arguments to STRANDS.

To take the next step and also return STRANDS (which leverages the
actual benefit - not copying unmodified strand elements), vmods need
to allocate a struct strands on the workspace.

This commit adds a utility function for this common task.

Implementation note: A single WS_Alloc() with some pointer arithmetic
would suffice, but using two results in cleaner code.

bin/varnishd/cache/cache_vrt.c

@@ -180,6 +180,30 @@ VRT_GetHdr(VRT_CTX, VCL_HEADER hs)
 	return (p);
 }
 
+/*--------------------------------------------------------------------
+ * Alloc Strands with space for n elements on workspace
+ *
+ * Error handling is deliberately left to the caller
+ */
+
+struct strands *
+VRT_AllocStrandsWS(struct ws *ws, int n)
+{
+	struct strands *s;
+	const char **p;
+
+	s = WS_Alloc(ws, sizeof *s);
+	p = WS_Alloc(ws, n * sizeof *p);
+
+	if (s == NULL || p == NULL)
+		return (NULL);
+
+	s->n = n;
+	s->p = p;
+
+	return (s);
+}
+
 /*--------------------------------------------------------------------
  * Build STRANDS from what is essentially a STRING_LIST
  */

include/vrt.h

@@ -60,6 +60,7 @@
  *	VRT_l_resp_body() changed
  *	VRT_l_beresp_body() changed
  *	VRT_Format_Proxy() added	// transitional interface
+ *	VRT_AllocStrandsWS() added
  * 10.0 (2019-09-15)
  *	VRT_UpperLowerStrands added.
  *	VRT_synth_page now takes STRANDS argument
@@ -206,6 +207,9 @@ struct strands {
 	const char	**p;
 };
 
+struct strands * VRT_AllocStrandsWS(struct ws *, int);
+
+
 /*
  * VCL_BLOB:
  *