syslog: use PassCred=yes for the /dev/log socket

Both kmsg-syslogd and the real syslog service want to receive
SCM_CREDENTIALS. With socket activation it is too late to set
SO_PASSCRED in the services.

src/kmsg-syslogd.c

@@ -91,7 +91,7 @@ static int server_init(Server *s, unsigned n_sockets) {
         }
 
         for (i = 0; i < n_sockets; i++) {
-                int fd, one = 1;
+                int fd;
 
                 fd = SD_LISTEN_FDS_START+i;
 
@@ -106,9 +106,6 @@ static int server_init(Server *s, unsigned n_sockets) {
                         goto fail;
                 }
 
-                if (setsockopt(fd, SOL_SOCKET, SO_PASSCRED, &one, sizeof(one)) < 0)
-                        log_error("SO_PASSCRED failed: %m");
-
                 zero(ev);
                 ev.events = EPOLLIN;
                 ev.data.fd = fd;

units/syslog.socket

@@ -18,6 +18,7 @@ Wants=syslog.target
 [Socket]
 ListenDatagram=/dev/log
 SocketMode=0666
+PassCred=yes
 
 # The service we activate on incoming traffic is
 # systemd-kmsg-syslogd.service. That doesn't mean however, that this