[Tagging] "new" highway tag for small and informal footpaths; trail

[SURLY_ru]

> highway=path is indeed what would currently be used for informal
> footpaths. But it can also be used to describe intentionally built, well
> maintained and paved ways.

Intentionally built way, too narrow for 4-wheel vehicles, is 
"highway=footway". Unintentional, informal way is "highway=path".
I see no needs for "highway=trail"