massive updates to multibyte/wide character support; deal with multibyte

massive updates to multibyte/wide character support; deal with multibyte
characters and strings instead of wide characters and strings as much as
possible, and move multibyte/wide character-specific functions into
their own source file, chars.c

git-svn-id: svn://svn.savannah.gnu.org/nano/trunk/nano@2248 35c25a1d-7b9e-4130-9fde-d3aeb78583b8