Separate MTU interface values for IPv4 and IPv6
Some ISPs do not offer IPv6 connetivity yet or offer it via a tunnelling mechanism.
As such, in some cases PMTU discovery can fail and it is sometimes necessary to explicitly define a lower MTU interface value.
Unfortunately, due to the 20 bytes overhead of a 6in4 tunnel an IPv6 MTU value of IPv4MTU - 20 bytes is necessary.
Shrinking the MTU to accommodate IPv6 results in poor IPv4 connectivity and vise versa.
Separate MTU interface values in the web GUI are proposed for IPv4 and IPv6 routes.