more fixes for files that don't end in magiclines: make cut_line() cut

more fixes for files that don't end in magiclines: make cut_line() cut
the text of the current line (if any), minus the nonexistent newline,
when we're on the last line of the file, and make sure again that the
file isn't marked as modified if the magicline is deleted and we're
supposed to have one, as it's more consistent that way (a marked cut of
the magicline adds a newline to the cutbuffer, while deleting the
magicline adds nothing)

git-svn-id: svn://svn.savannah.gnu.org/nano/trunk/nano@3114 35c25a1d-7b9e-4130-9fde-d3aeb78583b8