maint: avoid compiler warnings introduced by i18n patch
    
    * src/cut.c (convfail,CONVFAIL): Change to bool to avoid a GCC
    warning about signed vs. unsigned.  Also avoid "set but not used"
    of convfail in cut_characters_or_cut_bytes_no_split.
    (cut_fields_mb): Fix indentation.
    
    * src/expand.c: Add include for wctype.h to declare isblank().
    
    * src/fold.c (fold_file): Add "const" attribute to 'filename' parameter
    to avoid "const cast away" in call in main().
    
    * src/join.c (xfields_multibyte): Remove unused variable 't'.
    (keycmp): Evaluate the return value of wcrtomb().  Furthermore,
    when copying 'beg' to 'copy' array elements, remove the unnecessary
    cast to unsigned.
    (main): When assigning the newline string to 'newtab', cast away the
    implicit const.
    
    * src/pr.c: Remove the unneeded include of wctype.h and the define
    of iswprint().
    (print_stored): Add the unsigned qualifier to 'first' and 'last' to
    avoid a signedness warning.
    
    * src/sort.c (getmonth_mb): Cast away the 'const' qualifier of 's'
    when assigning to *ea.
    
    * src/uniq.c (different_multi): Evaluate the return result of wcrtomb();
    Add the include of assert.h for that.