remove ugly workarounds for most cases of edittop's or current's being
NULL (as those cases no longer occur due to the overhaul), and remove
detection of binary files (since it wasn't always accurate and will only
cause problems when UTF-8 support is added); also add a few minor fixes
of mine


git-svn-id: svn://svn.savannah.gnu.org/nano/trunk/nano@1928 35c25a1d-7b9e-4130-9fde-d3aeb78583b8