add DB's changes to titlebar() (with a few tweaks of mine) to make it

add DB's changes to titlebar() (with a few tweaks of mine) to make it
handle shorter screen widths better and display the filename using
display_string(), my strnlen() equivalent so the new titlebar() will
work on systems without strnlen(), and a few of DB's other minor changes
to make other display-related functions handle shorter screen widths
better, among other things

git-svn-id: svn://svn.savannah.gnu.org/nano/trunk/nano@1754 35c25a1d-7b9e-4130-9fde-d3aeb78583b8