remove display_string_len() again for the time being, since it's still

remove display_string_len() again for the time being, since it's still
prone to off-by-one errors that lead to memory corruption and I can't
find anything obviously wrong with it; always allocate enough space for
len columns of text now; any extra space that isn't used will be removed
by the null_at() call at the end of display_string() anyway

git-svn-id: svn://svn.savannah.gnu.org/nano/trunk/nano@2245 35c25a1d-7b9e-4130-9fde-d3aeb78583b8