MINOR: proxy: add proxy_get_next_id() to find next free proxy ID

This was previously achieved via the generic get_next_id() but we'll soon
get rid of generic ID trees so let's have a dedicated proxy_get_next_id().

include/haproxy/proxy.h

@@ -57,6 +57,7 @@ void free_proxy(struct proxy *p);
 const char *proxy_cap_str(int cap);
 const char *proxy_mode_str(int mode);
 const char *proxy_find_best_option(const char *word, const char **extra);
+uint proxy_get_next_id(uint from);
 void proxy_store_name(struct proxy *px);
 struct proxy *proxy_find_by_id(int id, int cap, int table);
 struct proxy *proxy_find_by_name(const char *name, int cap, int table);

src/cfgparse.c

@@ -2850,7 +2850,7 @@ init_proxies_list_stage1:
 			 * build or config options and we don't want them to
 			 * possibly reuse existing IDs.
 			 */
-			next_pxid = get_next_id(&used_proxy_id, next_pxid);
+			next_pxid = proxy_get_next_id(next_pxid);
 			curproxy->conf.id.key = curproxy->uuid = next_pxid;
 			eb32_insert(&used_proxy_id, &curproxy->conf.id);
 		}

src/proxy.c

@@ -538,6 +538,23 @@ const char *proxy_find_best_option(const char *word, const char **extra)
 	return best_ptr;
 }
 
+/* This function returns the first unused proxy ID greater than or equal to
+ * <from> in used_proxy_id. Zero is returned if no spare one is found (should
+ * never happen).
+ */
+uint proxy_get_next_id(uint from)
+{
+	struct eb32_node *used;
+
+	do {
+		used = eb32_lookup_ge(&used_proxy_id, from);
+		if (!used || used->key > from)
+			return from; /* available */
+		from++;
+	} while (from);
+	return from;
+}
+
 /* This function parses a "timeout" statement in a proxy section. It returns
  * -1 if there is any error, 1 for a warning, otherwise zero. If it does not
  * return zero, it will write an error or warning message into a preallocated