s4:dsdb Handle backlinks for Windows 2000 level linked attributes

s4:dsdb Handle backlinks for Windows 2000 level linked attributes

This revives the code from 5964acfa741d691c0196f91c0796122ec025f177,
before tridge and I simplified this too much, and removed the Windows
2000 functional level linked attribute support.

By telling the linked_attributes module that repl_meta_data has
handled the links, we avoid a conflict for the new style (functional
level 2003 and above) linked attributes.  However, we still need
backlinks for 2000 style linked attributes, so this allows that code
in the linked_attributes module to be revived to handle those.

Andrew Bartlett