tweak a few functions to remove the assumption that the file always ends

tweak a few functions to remove the assumption that the file always ends
in a magicline, and to rely on fileage and filebot instead of NULL for
their checks to detect the top or bottom of the file

git-svn-id: svn://svn.savannah.gnu.org/nano/trunk/nano@3080 35c25a1d-7b9e-4130-9fde-d3aeb78583b8