Properly handle device aliases used as dependencies

If a device unit has aliases defined in udev rules, and there are
other units that depend on that alias, as in

  BindTo=sys-subsystem-net-devices-eth0.device

then systemd will fail the start the alias, and any dependent units
will time out.  See

  https://bugs.freedesktop.org/show_bug.cgi?id=52580

This is because unit_add_name() in device_add_escaped_name() will
return EEXIST.

The solution taken here is to call device_update_unit() on the alias
name.  Thus if a unit with the alias name already exists, we reuse it;
otherwise a new unit is created.  Creating multiple units for a single
device is perhaps suboptimal, but it's consistent with the treatment
of udev symlinks in device_process_new_device().

src/core/device.c

@@ -269,11 +269,8 @@ static int device_update_unit(Manager *m, struct udev_device *dev, const char *p
                                         log_warning("SYSTEMD_ALIAS for %s is not a path, ignoring: %s", sysfs, e);
                                         free(e);
                                 } else {
-
+                                        device_update_unit(m, dev, e, false);
-                                        r = device_add_escaped_name(u, e);
                                         free(e);
-                                        if (r < 0)
-                                                goto fail;
                                 }
                         }
                 }