The rclone and sftp backends require starting a child process. The are
first moved into the foreground and back into the background after the
initial startup is done.
However, this behavior is also active if restic itself is started in the
background. In this case, restic changing the foreground process may
confuse the shell and in case of bash causes it to exit. Thus, disable
modification of the controlling process group of the tty if restic is
run in the background.
This removes code that is only used within a backend implementation from
the backend package. The latter now only contains code that also has
external users.
