gui: Call waitpid() from exec and status task.

gui: Call waitpid() from exec and status task.

This commit changes signal_post_select() to not call waitpid() via
para_reap_child() any more. Instead it notifies all tasks using the
new error code E_GUI_SIGCHLD. The exec task and the status task honor
this notification and call waitpid() for their "own" pid to check
whether the executing process or the stat process has terminated and
to obtain the exit status.

Although neither task cares deeply about the exit code, the menu
commands, which will be implemented in subsequent patches, will
care. So it's good to have the option to get this information.