Moved cp437_unicode_tbl and unicode_is_zerowidth() to (new file) unicode.c.

New function (derived from sbbs_t::utf8_to_cp437()): unicode_to_cp437()
New utf8 functions: utf8_replace_chars(), utf8_str_is_valid().
utf8_getc() enhancement: val arg may be NULL (for length/validation uses).

Convert quoted UTF-8 message text to CP437 when terminal is not UTF8.

Set Fido CHRS: UTF-8 header field when posted message is UTF-8.