journal: log filtering options support in PID1

Define new unit parameter (LogFilterPatterns) to filter logs processed by
journald.

This option is used to store a regular expression which is carried from
PID1 to systemd-journald through a cgroup xattrs:
`user.journald_log_filter_patterns`.

src/shared/bus-unit-util.c

@@ -1218,6 +1218,16 @@ static int bus_append_execute_property(sd_bus_message *m, const char *field, con
                 return 1;
         }
 
+        if (streq(field, "LogFilterPatterns")) {
+                r = sd_bus_message_append(m, "(sv)", "LogFilterPatterns", "a(bs)", 1,
+                                          eq[0] != '~',
+                                          eq[0] != '~' ? eq : eq + 1);
+                if (r < 0)
+                        return bus_log_create_error(r);
+
+                return 1;
+        }
+
         if (STR_IN_SET(field, "StandardInput",
                               "StandardOutput",
                               "StandardError")) {