Expose http_IsHdr(), it is useful to iterate over particular headers.

bin/varnishd/cache/cache.h

@@ -585,6 +585,7 @@ void HTTP_Clone(struct http *to, const struct http * const fm);
 void HTTP_Dup(struct http *to, const struct http * const fm);
 struct http *HTTP_create(void *p, uint16_t nhttp, unsigned);
 const char *http_Status2Reason(unsigned, const char **);
+int http_IsHdr(const txt *hh, hdr_t hdr);
 unsigned http_EstimateWS(const struct http *fm, unsigned how);
 void http_PutResponse(struct http *to, const char *proto, uint16_t status,
     const char *response);

bin/varnishd/cache/cache_http.c

@@ -423,7 +423,7 @@ http_PutField(struct http *to, int field, const char *string)
 
 /*--------------------------------------------------------------------*/
 
-static int
+int
 http_IsHdr(const txt *hh, hdr_t hdr)
 {
 	unsigned l;