Port or Parent MTU might be too low

[haslersn]

The role should give an error in the following cases:

• A bridge's parent has a lower MTU than the bridge
• A bridge's port has a lower MTU than the bridge

In case a physical port doesn't have an MTU set explicitly but is connected to at least one bridge, then I propose that it should get the maximum MTU of those bridges.

[dasJ]

Automatically bumping this issue because it was inactive for 57 days.

[dasJ]

Automatically bumping this issue because it was inactive for 223 days.