simplify code: use readStyle were possible

git-svn-id: http://svn.mkgmap.org.uk/mkgmap/trunk@2579 25d90789-57f7-4ee0-8453-03a3dfeeeb22
- correct check-styles: overlays are not used for points and polygons
- separate code for check-styles and list-styles option, check only a single style if --style was used.

git-svn-id: http://svn.mkgmap.org.uk/mkgmap/trunk@2580 25d90789-57f7-4ee0-8453-03a3dfeeeb22