journalctl: support --lines=+N for showing the oldest N entries

journalctl: support --lines=+N for showing the oldest N entries

After f58269510727964cb5c10e7d2f9849c442ea1f80, the wrong behavior
occurred when --since= and --lines= are both specified is fixed.
However, it seems that the old behavior is already being somewhat
widely used, and the function itself makes sense, i.e. to allow --lines=
to output the first N journal entries.

Therefore, let's support prefixing the number for --lines= with '+',
and provide such functionality.

Related: #28746