[Tagging] Feature: "covered=yes" - Added to Map Features Properties

[Randy]

I've added the key/value "covered=yes" to the Map Features property 
section. The purpose is to provide for tagging of nodes/ways/areas which 
are covered by another object, such as a building, where it is not 
appropriate to designate them at different layers.

It may also be used to designate buried objects, such as a buried power 
line or buried water main, where these are not in a tunnel or conduit. Or, 
to tag a buried area, such as an underground reservoir or cistern, or 
possibly an underground parking lot, where there is no other structure 
completely covering it to provide a different layer.

I researched "covered" and "buried" in the wiki, and didn't run across any 
conflicts except for a Belgian reference to "man_made=reservoir_covered", 
which would indicate a single case of redundancy.

-- 
Randy