The hash of running servers is now a hash of hashes which for each running

server holds not only its two main pids, but also the pidfile of the test
server and the 'slavepidfiles' for ftp* servers. This allows a better control
when stopping servers.

Now from runtests.pl when test servers are stopped they are signalled in
sequence TERM, INT and KILL allowing time in between for them to die. This
will give us a chance of gracefully stopping test servers, which we didn't
have when we were killing them in first instance.