fix #1819: fork_worker: ensure sync'ed workers control terminal

Use setpgid + tcsetpgrp instead of setsid if $sync (invocation via
cli), thus keeping /dev/tty - ssh-copy-id/ssh need it to read the
password, and putting the child in the forground. Further, ignore
SIGTTOU in child process (otherwise it gets stopped upon tcsetpgrp)

Signed-off-by: Stoiko Ivanov <s.ivanov@proxmox.com>
Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>