journald: do not strip leading whitespace from messages

Keep leading whitespace for compatibility with older syslog
implementations.  Also useful when piping formatted output to the
`logger` command.  Keep removing trailing whitespace.

Tested with `pstree | logger` and checking that the output of
`journalctl | tail` included aligned and formatted output.

Confirmed that all test cases still pass as expected.

(cherry picked from commit ec5ff444)