Validation tests for foot/footwat/sidewalk

[naoliv]

First a problem: validating a way with highway=foot + foot=no gives us a footway used with foot= (note that there isn't a value after the =)

Now the enhancements: highway=residential + sidewalk=both + foot=no doesn't issue a warning (it's as contradictory as using highway=foot + foot=no)

Also, highway=residential + footway=both doesn't warns that footway is deprecated (and that sidewalk should be used instead)