Derive initial interface selection based on policy

So far, any output module with a selection capability defaulted to
the first interface in the list as first pick. This uses the policy
configuration instead and thus allows to select which interface to
display first:

Examples:
 bmon -p 'em1,*'
 bmon -p 'eth*,lo,*'

The first rule that finds a matching interface is used and will
trigger initial selection.

Note that this is *ONLY* evaluated after the first read of the
statistics so if a more preferred interface appears later, the
selection will not be changed.

Signed-off-by: Thomas Graf <tgraf@suug.ch>

NEWS

@@ -1,3 +1,6 @@
+HEAD
+ * Pick default selected interface based on policy
+
 v3.9 - Jul 19, 2016
  * Color support
  * Add ability to reset statistics from curses UI

src/element.c

@@ -358,6 +358,26 @@ int element_set_usage_attr(struct element *e, const char *usage)
 	return 0;
 }
 
+void element_pick_from_policy(struct element_group *g)
+{
+	if (!list_empty(&allowed)) {
+		struct policy *p;
+
+		list_for_each_entry(p, &allowed, p_list) {
+			struct element *e;
+
+			list_for_each_entry(e, &g->g_elements, e_list) {
+				if (match_mask(p, e->e_name)) {
+					g->g_current = e;
+					return;
+				}
+			}
+		}
+	}
+
+	element_select_first();
+}
+
 struct element *element_current(void)
 {
 	struct element_group *g;
@@ -365,8 +385,12 @@ struct element *element_current(void)
 	if (!(g = group_current()))
 		return NULL;
 
+	/*
+	 * If no element is picked yet, pick a default interface according to
+	 * the selection policy.
+	 */
 	if (!g->g_current)
-		element_select_first();
+		element_pick_from_policy(g);
 
 	return g->g_current;
 }