MINOR: server: treat proxy server tree as without duplicate

MINOR: server: treat proxy server tree as without duplicate

Previous commit changes the unicity checks on server name inside a
proxy. This is now done directly during the parsing, prior to the tree
insertion.

Thus, the tree can now be considered without duplicate. Replace cebis_*
function by their equivalent cebuis_*. Also, insertion is now performed
via utility function _srv_register_name() which contains a new BUG_ON()
to ensure server name unicity principle is not violated.