vty cfg: make changing startup defaults of applications backwards compatible
Sometimes our recommendation of what the usual/good setting should be for a given vty config item changes.
Then we have the problem that we cannot change it without endangering running installations,
and/or creating hassle for admins.
So how about a general mechanism in all of our config files in form of a vty command, maybe like this:
so a user can decide when to make the cfg upgrade, independently from binary upgrades.
When 'defaults' is not specified, we would have to assume like 'defaults v0'.
(A bit more convenient for new users would be to add the newest available 'defaults' setting automatically somehow,
but I guess that also breaks the entire feature then, unless someone can think of an elegant way.)