journal: filter log based on LogFilterPatterns

journal: filter log based on LogFilterPatterns

Use LogFilterPatterns from the unit's cgroup xattr in order to keep or
discard log messages before writing them to the journal.
When a log message is discarded, it won't be written to syslog, console...
either.

When a native, syslog, or standard output log message is received,
systemd-journald will process it if it matches against at least one
allowed pattern (if any) and none of the denied patterns (if any).