Move pln_oninit(), lnd_oninit(), nuk_oninit() to filetable.c

They set up invariants, and thus should be always active, not just in
the server.  Since ef_blank() isn't used for these files outside the
server right now, this isn't a bug fix, just cleanup.

src/lib/subs/plane.c

@@ -87,14 +87,6 @@ pln_prewrite(int n, void *old, void *new)
 	unit_update_cargo((struct empobj *)pp);
 }
 
-void
-pln_oninit(void *ptr)
-{
-    struct plnstr *pp = ptr;
-
-    pp->pln_ship = pp->pln_land = -1;
-}
-
 char *
 prplane(struct plnstr *pp)
 {