PR #5358 reintroduced a version of the TIOCGPGRP ioctl call that works
on all Unix platforms except Linux, due to a bug/inconsistency in
x/sys/unix. This commit fixes that by introducing termstatus.Tcgetpgrp.
It also introduces termstatus.Getpgrp and termstatus.Tcsetpgrp to deal
with the different signature of unix.Getpgrp in Solaris vs. all other
Unix platforms and an int-overflowing constant on AIX, so that some
AIX/Solaris-specific code can be removed elsewhere and
foreground/background detection is done the same everywhere except on
Windows.
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.
