testnode: make killall support processes with changed title

testnode uses ProcessManager.killall to terminate all processes from a
path. To determine if a process is from a path, it looks at the command
line. This does not work for processes using setproctitle to change
their command line.

We can see in ps:

    $ ps -edf | grep nginx
    slapuse+ 115059  45574  0 16:14 ?        00:00:00 nginx: master process /srv/slapgrid/slappart46/t/cvt/i/0/tmp/shared/nginx/6d79cb0e7d81dce1be97eec8a5712f08/sbin/nginx -c /srv/slapgrid/slappart46/t/cvt/i/0/tmp/inst/T-0/etc/nginx-master-introspection.conf
    slapuse+ 115090 115059  0 16:14 ?        00:00:00 nginx: worker process

or by looking at cmdline, which is what psutil.Process.cmdline is using:

    $ cat /proc/115090/cmdline
    nginx: worker process

and that's why sometimes when cancelling a software release test while
it is running tests from a software using nginx, some processes are
leaked, they keep using the port and next test running on this testnode
fail.

In that case, killall is called with /srv/slapgrid/slappart46/t/cvt , we
can not find such process with cmdline, but we can extend this heuristic
to use the current working directory:

    $ ls -al /proc/115090/cwd
    lrwxrwxrwx 1 slapuser46 slapuser46 0 Oct 19 16:16 /proc/115090/cwd -> /srv/slapgrid/slappart46/t/cvt/i/0/tmp/inst/T-0

This also applies an optimization of only considering processes of the
current unix user.