MINOR: proxy: Store orgto_hdr_name as a `struct ist`

The orgto_hdr_name is already processed as an ist in `http_process_request`,
lets also just store it as such.

see 0643b0e7e ("MINOR: proxy: Make `header_unique_id` a `struct ist`") for a
very similar past commit.
diff --git a/include/haproxy/proxy-t.h b/include/haproxy/proxy-t.h
index 8277c09..805e1b4 100644
--- a/include/haproxy/proxy-t.h
+++ b/include/haproxy/proxy-t.h
@@ -353,8 +353,7 @@
 	struct net_addr except_xff_net;         /* don't x-forward-for for this address. */
 	struct net_addr except_xot_net;         /* don't x-original-to for this address. */
 	struct ist fwdfor_hdr_name;			/* header to use - default: "x-forwarded-for" */
-	char *orgto_hdr_name;			/* header to use - default: "x-original-to" */
-	int orgto_hdr_len;			/* length of "x-original-to" header */
+	struct ist orgto_hdr_name;			/* header to use - default: "x-original-to" */
 	char *server_id_hdr_name;                   /* the header to use to send the server id (name) */
 	int server_id_hdr_len;                      /* the length of the id (name) header... name */
 	int conn_retries;			/* maximum number of connect retries */
diff --git a/src/cfgparse-listen.c b/src/cfgparse-listen.c
index d858c34..121f1de 100644
--- a/src/cfgparse-listen.c
+++ b/src/cfgparse-listen.c
@@ -2399,11 +2399,10 @@
 
 			curproxy->options |= PR_O_ORGTO;
 
-			free(curproxy->orgto_hdr_name);
-			curproxy->orgto_hdr_name = strdup(DEF_XORIGINALTO_HDR);
-			if (!curproxy->orgto_hdr_name)
+			istfree(&curproxy->orgto_hdr_name);
+			curproxy->orgto_hdr_name = istdup(ist(DEF_XORIGINALTO_HDR));
+			if (!isttest(curproxy->orgto_hdr_name))
 				goto alloc_error;
-			curproxy->orgto_hdr_len  = strlen(DEF_XORIGINALTO_HDR);
 			curproxy->except_xot_net.family = AF_UNSPEC;
 
 			/* loop to go through arguments - start at 2, since 0+1 = "option" "originalto" */
@@ -2441,11 +2440,10 @@
 						err_code |= ERR_ALERT | ERR_FATAL;
 						goto out;
 					}
-					free(curproxy->orgto_hdr_name);
-					curproxy->orgto_hdr_name = strdup(args[cur_arg+1]);
-					if (!curproxy->orgto_hdr_name)
+					istfree(&curproxy->orgto_hdr_name);
+					curproxy->orgto_hdr_name = istdup(ist(args[cur_arg+1]));
+					if (!isttest(curproxy->orgto_hdr_name))
 						goto alloc_error;
-					curproxy->orgto_hdr_len  = strlen(curproxy->orgto_hdr_name);
 					cur_arg += 2;
 				} else {
 					/* unknown suboption - catchall */
diff --git a/src/http_ana.c b/src/http_ana.c
index f02b844..8371148 100644
--- a/src/http_ana.c
+++ b/src/http_ana.c
@@ -711,8 +711,7 @@
 	 */
 	if ((sess->fe->options | s->be->options) & PR_O_ORGTO) {
 		const struct sockaddr_storage *dst = si_dst(cs_si(s->csf));
-		struct ist hdr = ist2(s->be->orgto_hdr_len ? s->be->orgto_hdr_name : sess->fe->orgto_hdr_name,
-				      s->be->orgto_hdr_len ? s->be->orgto_hdr_len  : sess->fe->orgto_hdr_len);
+		struct ist hdr = isttest(s->be->orgto_hdr_name) ? s->be->orgto_hdr_name : sess->fe->orgto_hdr_name;
 
 		if (dst && dst->ss_family == AF_INET) {
 			/* Add an X-Original-To header unless the destination IP is
diff --git a/src/proxy.c b/src/proxy.c
index b48c24e..33328e2 100644
--- a/src/proxy.c
+++ b/src/proxy.c
@@ -1440,7 +1440,7 @@
 	ha_free(&defproxy->defbe.name);
 	ha_free(&defproxy->conn_src.iface_name);
 	istfree(&defproxy->fwdfor_hdr_name);
-	ha_free(&defproxy->orgto_hdr_name); defproxy->orgto_hdr_len = 0;
+	istfree(&defproxy->orgto_hdr_name);
 	ha_free(&defproxy->server_id_hdr_name); defproxy->server_id_hdr_len = 0;
 
 	list_for_each_entry_safe(acl, aclb, &defproxy->acl, list) {
@@ -1604,10 +1604,8 @@
 	if (isttest(defproxy->fwdfor_hdr_name))
 		curproxy->fwdfor_hdr_name = istdup(defproxy->fwdfor_hdr_name);
 
-	if (defproxy->orgto_hdr_len) {
-		curproxy->orgto_hdr_len  = defproxy->orgto_hdr_len;
-		curproxy->orgto_hdr_name = strdup(defproxy->orgto_hdr_name);
-	}
+	if (isttest(defproxy->orgto_hdr_name))
+		curproxy->orgto_hdr_name = istdup(defproxy->orgto_hdr_name);
 
 	if (defproxy->server_id_hdr_len) {
 		curproxy->server_id_hdr_len  = defproxy->server_id_hdr_len;