chgserver: create new process group after fork (issue5051) This is to make SIGTSTP work. Before the patch, the server process group is considered "orphaned" and will ignore SIGTSTP, SIGTTIN, SIGTTOU, according to POSIX. See the comment above `will_become_orphaned_pgrp` in `kernel/exit.c` from Linux 4.3 for details. SIGTSTP is important if chgserver runs some ncurses commend like `commit -i`. Ncurses has its own SIGTSTP handler which will do the following: 1. Clean the screen 2. Stop itself by resending SIGTSTP to itself 3. Restore the screen If SIGTSTP is ignored, step 2 will be a noop, which means the process cannot be suspended properly. In order to make things work, chg client needs to forward SIGTSTP and SIGCONT to server as well.