gui: Kill also child processes of external commands.

gui: Kill also child processes of external commands.

When a command is executed as a display command from para_gui, and
the command is interrupted from within para_gui by pressing any key,
para_gui only kills the spawned process but leaves alone its child
processes.

Address this problem by putting the spawned process into a separate
process group so that we can easily kill all processes of the group
by passing the negated PID of the child to kill(2).

In theory, the change in exec.c also affects para_mixer, which executes
para_client and para_audioc. However, para_mixer never kills any of
its child processes.