Aliases pointing within the original search scope could cause
the same entry to be returned multiple times. Check for subtree
scope and duplicate scopes.

Non-recursive tree walker.

update mdb_tool_idl_add prototype

Do not access a size_t via an unsigned long pointer, an MDB_val via a
berval pointer, or the inverse. Instead copy into the proper data types.

Tighten up entry_alloc/entry_decode
Track parent nodes in idscopes