journalctl: add option to forgo ellipsization

journalctl: add option to forgo ellipsization

Sometimes it is better to see messages in full, and the existing
set of options didn't allow this easily. E.g. now

  journalctl -f --full

will behave like

  tail -f /var/log/messages

of yore.

Long option only for now, since small letters are becoming
scarce, and this doesn't feel like a capital-letter-option.
'-u' would be nice, and the above command would be spelled

  journalctl -fu