journal: store the original syslog input as SYSLOG_RAW=

journal: store the original syslog input as SYSLOG_RAW=

This allows the original stream to be recreated and/or verified. The new field
is written if any stripping was done or if the input message contained embeded
NULs.

$ printf '<13>Sep 15 15:07:58 HOST: x\0y' | nc -w1 -u -U /run/systemd/journal/dev-log

$ journalctl -o json-pretty ...
{
  ...
  "MESSAGE" : "x",
  "SYSLOG_RAW" : [ 60, 49, 51, 62, 83, 101, 112, 32, 49, 53, 32, 49, 53, 58, 48, 55, 58, 53, 56, 32, 72, 79, 83, 84, 58, 32, 120, 0, 121 ]
}

$ journalctl -o export ... | cat -v
...
MESSAGE=x
SYSLOG_RAW
^]^@^@^@^@^@^@^@<13>Sep 15 15:07:58 HOST: x^@y

This mostly fixes #4863.