systemctl: show hint about --full when lines don't fit
logs-show: fix indentation for 2nd and later lines, show lines in full

Now --full will show long fields in full, like it already did
with --all.
logs-show: limit to 3 lines and use dots if not showing full message

So far, we would show up to 128 bytes from a message, simply
cutting of the rest. With multiline messages, it is quite common
for a message to be longer than that, and this model doesn't really
work anymore.

A new limit is added: up to 3 lines will be shown, unless --full is
used (c.f. first line below).  The limit for bytes is extended to 300
bytes. An ellipsis will always be used, if some form of truncation
occurs. If the tail of the message is cut off, either because of
length or line limit, dots will be shown at the end of the last
line. If this last line is short, the dots will be simply appended. If
the last line is too long for that, it will be ellipsized with dots at
the very end.

Note that the limits are in bytes, not characters, and we suck at
outputting unicode strings (c.f. last three lines below).

Aug 11 10:46:21 fedora python[67]: test message
                                   line
                                   line...
Aug 11 10:50:47 fedora python[76]: test message word word word word word word word word word word word wor...
Aug 11 10:55:11 fedora python[83]: xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx...
Aug 11 11:03:21 fedora python[90]: ąąąąąąąąąąąąąąąąąąąąąąąąąąąąąą...
Aug 11 11:03:53 fedora python[97]: aąąąąąąąąąąąąąąąąąąąąąąąąąąąąąą...
Aug 11 11:25:45 fedora python[121]: aąąąąąąąąąąąąąąąąąąąąąąąąąąąąąąąąąą�...