Detect that a child is waiting for input

[unDEFER via Digitalmars-d]

Here is good information about difference between foreground 
process groups and background:
https://www.win.tue.nl/~aeb/linux/lk/lk-10.html

Shortly, there is only one process group which is foreground, you 
can get it with tcgetpgrp(fd) or set it with tcsetpgrp(fd,pgrp). 
To setup process group of the process use setpgid(pid, pgid);