Add a unique_list function

The unique_list function takes a list as parameter, and returns
a list without any duplicated elements. The order of the elements
is maintained (i.e. if the first occurrence of an element appears in
the list before the first occurrence of another, the former will
precede the latter in the final list).

pan/functions.pan

@@ -218,3 +218,21 @@ function full_hostname_from_object = {
     return (OBJECT);
 };
 
+
+@documentation{
+    desc = Function to deduplicate the content of a list, only the first \
+        occurence of each element in the list will be kept in the final \
+        list returned to the caller
+}
+function unique_list = {
+    to_dict = dict();
+    to_list = list();
+    foreach(idx; item; ARGV[0]) {
+        itemid = to_string(item);
+        if (!exists(to_dict[escape(itemid)])) {
+            append(to_list, item);
+            to_dict[escape(itemid)] = 1;
+        };
+    };
+    return(to_list);
+};