journalctl: fix output when --since is used with --lines

journalctl: fix output when --since is used with --lines

Before this commit, if --since is used with --lines=N,
we seek to the place of --since and search afterwards
there, resulting in outputing the first N lines.

After this commit, we only do the above if --since is used without
--reverse and --lines. Otherwise we seek to the tail first and check
if the entry is within the range of --since later.