kernel-pfroute: Set lower MTU on TUN devices

kernel-pfroute: Set lower MTU on TUN devices

The default MTU of 1500 is too high if kernel-libipsec is used (considering
the overhead of UDP-encapsulated ESP), but might also have an effect if
a TUN device is only used to install a virtual IP (the route points to it,
so the system might use its MTU and 1500 would still be too high).

This also works around an issue on macOS 12 where no RTM_IFINFO event
is sent for the newly created TUN device (neither for the creation,
setting it "up", nor adding the address).  Changing the MTU, however,
triggers such an event and we can detect the virtual IP.

Closes strongswan/strongswan#707