netfilter: netns ebtables: part 2

* return ebt_table from ebt_register_table(), module code will save it into
  per-netns data for unregistration
* duplicate ebt_table at the very beginning of registration -- it's added into
  list, so one ebt_table wouldn't end up in many lists (and each netns has
  different one)
* introduce underscored tables in individial modules, this is temporary to not
  break bisection.

Signed-off-by: Alexey Dobriyan <adobriyan@gmail.com>
Signed-off-by: Patrick McHardy <kaber@trash.net>