pass-through stdin fd instead of content if not a terminal

pass-through stdin fd instead of content if not a terminal

Commit 299aea924ccef428219ed6f1a026c122678429e6 fixes the problem of
not logging terminal in case stdin & stdout are not a terminal. The
problem is that we are then trying to pass-through stdin content by
reading from the apt-process stdin and writing it to the stdin of the
child (dpkg), which works great for users who can control themselves,
but pipes and co are a bit less forgiving causing us to pass everything
to the first child process, which if the sending part of the pipe is
e.g. 'yes' we will never see the end of it (as the pipe is full at some
point and further writing blocks).

There is a simple solution for that of course: If stdin isn't a terminal,
we us the apt-process stdin as stdin for the child directly (We don't do
this if it is a terminal to be able to save the typed input in the log).

Closes: 773061