journalctl: add new output mode "short-full" (#3880)

journalctl: add new output mode "short-full" (#3880)

This new output mode formats all timestamps using the usual format_timestamp()
call we use pretty much everywhere else. Timestamps formatted this way are some
ways more useful than traditional syslog timestamps as they include weekday,
month and timezone information, while not being much longer. They are also not
locale-dependent. The primary advantage however is that they may be passed
directly to journalctl's --since= and --until= switches as soon as #3869 is
merged.

While we are at it, let's also add "short-unix" to shell completion.