Fix interface of Intrusive List

(Reported by Brian Sipos):
* incorrect clear operator usage
* useless prototype of _insert, inconsistent with the policy of other functions
* syntax error in _insert

example/ex-i-list.c

@@ -0,0 +1,56 @@
+#include "m-i-list.h"
+
+typedef struct data_s {
+    M_ILIST_INTERFACE(list1, struct data_s);
+    char *ptr;
+    int  n;
+} data_t;
+
+static void
+data_init(data_t *p)
+{
+    M_ILIST_INIT_FIELD(list1, *p);
+    p->ptr = malloc(100);
+    p->n = 100;
+}
+
+static void
+data_clear(data_t *p)
+{
+    free(p->ptr);
+    p->n = 0;
+}
+
+static void
+data_init_set(data_t *p, data_t s)
+{
+    p->ptr = malloc(s.n);
+    p->n = s.n;
+    memcpy(p->ptr, s.ptr, s.n);
+}
+
+#define DATA_OP (INIT(API_2(data_init)), INIT_SET(API_2(data_init_set)), CLEAR(API_2(data_clear)) )
+
+M_ILIST_DEF(list1, data_t, DATA_OP)
+
+data_t pool[100];
+
+int main(void)
+{
+    list1_t l;
+
+    list1_init(l);
+    data_init(&pool[0]);
+    list1_push_back(l, &pool[0]);
+    data_init(&pool[1]);
+    list1_push_back(l, &pool[1]);
+
+    list1_it_t it;
+    list1_it(it, l);
+    data_init(&pool[2]);
+    list1_insert(l, it, &pool[2]);
+
+    list1_clear(l);
+
+    exit(0);
+}