steal preexec_fn trick from exabgp processes.py, fixes #183

Tested with `./healthcheck.py --ip 192.168.1.1 --ip 192.168.1.2 --no-syslog --no-ip-setup --cmd 'sleep 35' --withdraw-on-down --timeout 1`, this patch avoids the 'sleep 35' processes from piling up.