refactor(service,stop): improve PID management, use 'kill' command

* move '$cfname' and '$pidfile' to the top
* use arithmetic context

Signed-off-by: Edouard Lopez <dev+git@edouard-lopez.com>
refactor(service,start): define '$pidfile', use it in check_no_pid_exists()
refactor(service,start): revamp 'autossh' call, use '&' for background process

* put to background using '&' because '-f' doesn't behave as expected.
* remove overzealous 'bash' command and use '-c' alone
* move back exports inside the '-c' command