Revive struct empfile callback init as oninit

Commit a71f0158 removed unused callback init.  Bring it back renamed
to oninit, and without the redundant first argument.

include/file.h

@@ -60,6 +60,10 @@ struct empfile {
     /* flags bit EFF_CUSTOM also varies */
 
     /* User callbacks, may all be null */
+    /*
+     * Called after element initialization.  ELT is the element.
+     */
+    void (*oninit)(void *elt);
     /*
      * Called after read.  ID is the element id, and ELT is the
      * element read.  May modify the element.  Modifications are