ssize_t are different sizes.  Make filestruct->lineno a ssize_t (so that
we can avoid negative line numbers at the "Go To Line" prompt),
current_y a ssize_t (in order to hold the maximum difference between two
filestruct->lineno's), totlines a size_t, and change related variables
to match


git-svn-id: svn://svn.savannah.gnu.org/nano/trunk/nano@2782 35c25a1d-7b9e-4130-9fde-d3aeb78583b8