[Tagging] Clean up *=milestone (WAS: pk vs kp on *=milestone and default unit?)

[ppece]

Hi. I'm from polish OSM society. We're currently running local
competition to map milestones and we're using this scheme.

> * I would suggest to use distance= (mileage= is not metric) instead of
> pk/kp and km (kilometre) as default unit.

I agree with that. We were close to develope new tagging scheme with
"distance" key instead of "pk", but we eventually found this scheme in
wiki. We stick "pk" only beceuase it is already in use in some countries
like Germany. "pk" says nothing about the tag and (as far as i know) is
not related to english, but french. If there is opportunity to change,
"distance" key will be better than "pk" (kilometre as default unit).

ppece