tipc: Add support for customized subscription overlap handling

This patch enables TIPC's topology server code to do customized
overlap detection handling on a per-subscription basis.  (This
capability is needed to support the upcoming introduction of
multi-cluster TIPC networks.)

Signed-off-by: Allan Stephens <allan.stephens@windriver.com>
Signed-off-by: David S. Miller <davem@davemloft.net>