util: rename list_empty() to list_is_empty()

This makes it clear that it's a boolean test and not an action
(eg. "empty the list").

Reviewed-by: Eric Engestrom <eric@engestrom.ch>
diff --git a/src/util/list.h b/src/util/list.h
index a450228..7165f15 100644
--- a/src/util/list.h
+++ b/src/util/list.h
@@ -77,11 +77,11 @@
     list->prev = item;
 }
 
-static inline bool list_empty(const struct list_head *list);
+static inline bool list_is_empty(const struct list_head *list);
 
 static inline void list_replace(struct list_head *from, struct list_head *to)
 {
-    if (list_empty(from)) {
+    if (list_is_empty(from)) {
         list_inithead(to);
     } else {
         to->prev = from->prev;
@@ -106,7 +106,7 @@
     item->prev = item;
 }
 
-static inline bool list_empty(const struct list_head *list)
+static inline bool list_is_empty(const struct list_head *list)
 {
    return list->next == list;
 }
@@ -130,7 +130,7 @@
 
 static inline void list_splice(struct list_head *src, struct list_head *dst)
 {
-   if (list_empty(src))
+   if (list_is_empty(src))
       return;
 
    src->next->prev = dst;
@@ -141,7 +141,7 @@
 
 static inline void list_splicetail(struct list_head *src, struct list_head *dst)
 {
-   if (list_empty(src))
+   if (list_is_empty(src))
       return;
 
    src->prev->next = dst;