fs-utils: new wrapper fd_reopen_propagate_append_and_position()

fs-utils: new wrapper fd_reopen_propagate_append_and_position()

We may want to propagate O_APPEND, or (try to) keep the current file position,
even if we use fd_reopen() to re-initialize (and "unshare") other file
description status.

For now, used only with --pty to keep/propagate O_APPEND (and/or) position
if set on stdin/stdout.

If we re-open stdout and "drop" the O_APPEND,
we get rather "unexpected" behavior,
for example with repeated "systemd-run --pty >> some-log".

If someone carefully pre-positioned the passed in original file descriptors,
we avoid surprises if we do not reset file postition to zero.

fcntl F_GETFL first, and propagate O_APPEND if present in the existing flags.

Then use lseek to propagate the file position.