[Talk-us] boundary = national_park in the US

[Daniel Sabo]

boundary=national_park gets stretched around quite a bit mapping the US, it's being used for:
National Parks
National Forests
National Monuments
National Preserves
National Recreation Areas
State Parks
State Forests (not sure if any of these have actually been imported).

I've seen admin_level used to differentiate between state and national parks, but that doesn't distinguish between park/forest/monument/preserve.

There is also the proposed boundary=protected_area, but while that provides a nice international baseline it still won't describe what kind of park it is in a US context. Also, since they're both boundary tags they'll need to be on separate relations which I won't be able to render nicely without a preprocessor.

Would adding a national_park key be a good choice here? Does admin_level belong on this type of boundary?