redbug_dtop: don't crash on mnesia ext tables

[mikpe]

If you try to run redbug:dtop() in a system with mnesia ext tables, it crashes with a badarith because redbug_dtop:ets_size/1 multiplies the atom undefined with an integer.

This happens because get_term_storage_type/1 defaults to ets for not explicitly matched storage types. This is wrong for e.g. ext LevelDB tables: only disc_copies should be mapped to ets.

Also change the type marker remote_only to undefined to better match its semantics (anything dtop doesn't know how to handle).