Use shifted offsets in LBL so that more labels can be saved.

Labels are now stored with offset/2 so that you can hold twice
as many labels as previously. There is also a proper check for
overflowing the LBL section.


git-svn-id: http://svn.mkgmap.org.uk/mkgmap/trunk@2759 25d90789-57f7-4ee0-8453-03a3dfeeeb22