MEDIUM: http: merge acl and pattern header fetch functions

HTTP header fetch is now done using smp_fetch_hdr() for both ACLs and
patterns. This one also supports an occurrence number, making it possible
to specify explicit occurrences for ACLs and patterns.
diff --git a/include/proto/proto_http.h b/include/proto/proto_http.h
index 16f03c2..06fe292 100644
--- a/include/proto/proto_http.h
+++ b/include/proto/proto_http.h
@@ -98,7 +98,7 @@
 void http_capture_bad_message(struct error_snapshot *es, struct session *s,
                               struct http_msg *msg,
 			      int state, struct proxy *other_end);
-unsigned int http_get_hdr(struct http_msg *msg, const char *hname, int hlen,
+unsigned int http_get_hdr(const struct http_msg *msg, const char *hname, int hlen,
 			  struct hdr_idx *idx, int occ,
 			  struct hdr_ctx *ctx, char **vptr, int *vlen);