udev: introduce NAMING_STABLE_VIRTUAL_MACS (retroactively)

udev: introduce NAMING_STABLE_VIRTUAL_MACS (retroactively)

This is for 6d3646406560. It turns out that this is causing more problems than
expected. Let's retroactively introduce naming scheme v241 to conditionalize
this change.

Follow-up for #12792 and 6d36464065601f7. See also
https://bugzilla.suse.com/show_bug.cgi?id=1136600.

$ SYSTEMD_LOG_LEVEL=debug NET_NAMING_SCHEME=v240 build/udevadm test-builtin net_setup_link /sys/class/net/br11
$ SYSTEMD_LOG_LEVEL=debug NET_NAMING_SCHEME=v241 build/udevadm test-builtin net_setup_link /sys/class/net/br11
...
@@ -20,11 +20,13 @@
 link_config: could not set ethtool features for br11
 Could not set offload features of br11: Operation not permitted
 br11: Device has name_assign_type=3
-Using interface naming scheme 'v240'.
+Using interface naming scheme 'v241'.
 br11: Policy *keep*: keeping existing userspace name
 br11: Device has addr_assign_type=1
-br11: No stable identifying information found
-br11: Could not generate persistent MAC: No data available
+br11: Using "br11" as stable identifying information
+br11: Using generated persistent MAC address
+Could not set Alias=, MACAddress= or MTU= on br11: Operation not permitted
+br11: Could not apply link config, ignoring: Operation not permitted
 Unload module index
 Unloaded link configuration context.
 ID_NET_DRIVER=bridge