afprog: kill program destination before stopping syslog-ng

Stop procedure of syslog-ng is not stopped program destination and it is
running after syslog-ng was stopped. This patch set program group ID to all
child processes of syslog-ng and kill them process that have the same
program gorup ID than syslog-ng before syslog-ng has been stoped.

Signed-off-by: Budai Laszlo <Laszlo.Budai@balabit.com>