afprog: Set the process group of the created child

When forking off a child process, set its process group to its PID, so
killing the whole process group in deinit will work. This means that
programs started by the program() source or destination() are now
correctly terminated on reload and exit.

Reported-by: Ryan Frederick
Signed-off-by: Gergely Nagy <algernon@balabit.hu>