Set timestamp automatically on write

Do it in do_write().  Remove the setting of timestamps elsewhere.

This fixes empdump to set timestamps to the current time instead of
zero on import.

src/lib/subs/sect.c

@@ -68,8 +68,6 @@ sct_prewrite(int id, void *ptr)
     struct sctstr *sp = ptr;
     struct sctstr sect;
 
-    time(&sp->sct_timestamp);
-
     bridge_damaged(sp, NULL);
     checksect(sp);
     getsect(sp->sct_x, sp->sct_y, &sect);