Rename tag display_name to mkgmap:display_name

The tag display_name is not an official but an internally used tag of mkgmap and should therefore have the prefix "mkgmap:".
The old unprefixed tag is still accepted but a warning is printed that it is deprecated.

git-svn-id: http://svn.mkgmap.org.uk/mkgmap/trunk@2431 25d90789-57f7-4ee0-8453-03a3dfeeeb22