Remove as many instances of 'unsigned' as

Remove as many instances of 'unsigned' as
possible, as some of them were not conforming to ANSI C, and
they made the code hard to read.  Avoiding 'unsigned' cuts
down on the number of casts.

(newline_character, space_character, save_char):
Now char, not unsigned char.
(obuf): Now char *, not unsigned char *.
(ascii_to_ebcdic, ascii_to_ibm, ebcdic_to_ascii):
Now char[], not unsigned char[].
(translate_charset, translate_buffer, swab_buffer, skip, copy_simple,
copy_with_block, copy_with_unblock):
Arg now points to char, not unsigned char.
All callers changed.
(translate_charset, parse_conversion, apply_translations):
Use int index, not unsigned int.
(bit_count): Arg is now int, not unsigned int.
Callers not changed, as they already assumed this.
(translate_buffer): Cast char to unsigned char before using
it as a subscript.
(swab_buffer): Returns char *, not unsigned char *.
(dd_copy): Use char, not unsigned char, for buffers.
Use size_t for possibly-large index, not unsigned int.