patch 8.1.0614: placing signs can be complicated

Problem:    Placing signs can be complicated.
Solution:   Add functions for defining and placing signs.  Introduce a group
            name to avoid different plugins using the same signs. (Yegappan
            Lakshmanan, closes #3652)

src/misc2.c

@@ -894,6 +894,19 @@ alloc_clear(unsigned size)
     return p;
 }
 
+/*
+ * Same as alloc_clear() but with allocation id for testing
+ */
+    char_u *
+alloc_clear_id(unsigned size, alloc_id_T id UNUSED)
+{
+#ifdef FEAT_EVAL
+    if (alloc_fail_id == id && alloc_does_fail((long_u)size))
+	return NULL;
+#endif
+    return alloc_clear(size);
+}
+
 /*
  * alloc() with check for maximum line length
  */