Decided to use UNICODE code points rather than UTF-8 sequences for faster

UNICODE -> CP437 conversions (no string/memory compares needed), so
cp437_utf8_tbl.* is being replaced by cp437_unicode_tbl.*.

Added utf8 module which includes:
- utf8_normalize_str() - normalize US-ASCIIZ string, from mailsrvr.c
- utf8_putc() - encoder / length calculator
- utf8_getc() - decoder

The latter 2 were swiped from OpenSSL's a_utf8.c.