sd-journal: when formatting log messages, implicitly strip trailing whitespace

sd-journal: when formatting log messages, implicitly strip trailing whitespace

When converting log messages from human readable text into binary records to
send off to journald in sd_journal_print(), strip trailing whitespace in the
log message. This way, handling of logs made via syslog(), stdout/stderr and
sd_journal_print() are treated the same way: trailing (but not leading)
whitespace is automatically removed, in particular \n and \r. Note that in case
of syslog() and stdout/stderr based logging the stripping takes place
server-side though, while for the native protocol based transport this takes
place client-side. This is because in the former cases conversion from
free-form human-readable strings into structured, binary log records takes
place on the server-side while for journal-native logging it happens on the
client side, and after conversion into binary records we probably shouldn't
alter the data anymore.

See: #3416