when doing marked spell-check, properly handle the case where the mark

when doing marked spell-check, properly handle the case where the mark
ends in the middle of a word and that word is spell-checked; also move
the logic to detect an upside-down mark to mark_order() instead of using
the same logic in three different places, and rename a few variables for
consistency

git-svn-id: svn://svn.savannah.gnu.org/nano/trunk/nano@2070 35c25a1d-7b9e-4130-9fde-d3aeb78583b8