socket: add new Symlinks= option for socket units

With Symlinks= we can manage one or more symlinks to AF_UNIX or FIFO
nodes in the file system, with the same lifecycle as the socket itself.

This has two benefits: first, this allows us to remove /dev/log and
/dev/initctl from /dev, thus leaving only symlinks, device nodes and
directories in the /dev tree. More importantly however, this allows us
to move /dev/log out of /dev, while still making it accessible there, so
that PrivateDevices= can provide /dev/log too.

src/core/dbus-socket.c

@@ -108,6 +108,7 @@ const sd_bus_vtable bus_socket_vtable[] = {
         SD_BUS_PROPERTY("PassSecurity", "b", bus_property_get_bool, offsetof(Socket, pass_sec), SD_BUS_VTABLE_PROPERTY_CONST),
         SD_BUS_PROPERTY("RemoveOnStop", "b", bus_property_get_bool, offsetof(Socket, remove_on_stop), SD_BUS_VTABLE_PROPERTY_CONST),
         SD_BUS_PROPERTY("Listen", "a(ss)", property_get_listen, 0, SD_BUS_VTABLE_PROPERTY_CONST),
+        SD_BUS_PROPERTY("Symlinks", "as", NULL, offsetof(Socket, symlinks), SD_BUS_VTABLE_PROPERTY_CONST),
         SD_BUS_PROPERTY("Mark", "i", bus_property_get_int, offsetof(Socket, mark), SD_BUS_VTABLE_PROPERTY_CONST),
         SD_BUS_PROPERTY("MaxConnections", "u", bus_property_get_unsigned, offsetof(Socket, max_connections), SD_BUS_VTABLE_PROPERTY_CONST),
         SD_BUS_PROPERTY("MessageQueueMaxMessages", "x", bus_property_get_long, offsetof(Socket, mq_maxmsg), SD_BUS_VTABLE_PROPERTY_CONST),