add in a check/test to ensure that the backup node in a failover is a forwarding subscriber

If the backup node is not a forwarding subscriber to all of the sets from the origin
then the failover can't complete.  It is best to check this early on and warn the user
before the failover actually starts.

Branch
------
master

Details
-------
http://git.postgresql.org/gitweb?p=slony1-engine.git;a=commitdiff;h=60f0c17855831f3b57601f2fdb24e814db3aa366

Modified Files
--------------
clustertest/disorder/tests/Failover.js |   33 ++++++++++++++++++++++---
src/slonik/slonik.c                    |   42 +++++++++++++++++++++++++++++++-
2 files changed, 71 insertions(+), 4 deletions(-)