tcf-agent: kill with USR2 in systemd stop

tcf-agent: kill with USR2 in systemd stop

tcf-agent ignores SIGTERM, so upstream uses USR2 instead. This issue was noticed
by Jan Kiszka and Brian Avery around the same time:

https://patchwork.openembedded.org/patch/139546/
https://patchwork.openembedded.org/patch/139560/

However, these patches fixed only the init scripts, not the systemd service
file. This patch fixes the systemd file.

Signed-off-by: Martin Kelly <mkelly@xevo.com>
Signed-off-by: Ross Burton <ross.burton@intel.com>
Signed-off-by: Javier Viguera <javier.viguera@digi.com>
Signed-off-by: Armin Kuster <akuster808@gmail.com>