eventscripts: kill_tcp_connections() should send connections to stdin

eventscripts: kill_tcp_connections() should send connections to stdin

This avoids issuing multiple "ctdb killtcp" commands to terminate tcp
connections, one per connection.  This will considerably reduce the
time when there is a large number of tcp connections.  This also makes
it possible to avoid calling "ctdb killtcp" when there are no connections.

Add a couple of unit tests for killtcp and update eventscript unit
test infrastructure to support.

Signed-off-by: Martin Schwenke <martin@meltin.net>
Pair-programmed-with: Amitay Isaacs <amitay@gmail.com>