Replaced unicode_is_zerowidth() with unicode_width(), in preparation for
"fullwidth" char support.
Added UNICODE_UNDEFINED definition (0x0000)
(UNICODE_NULL is already defined, at least in MSVC).

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).

Most of the copyright years in the source code were misleading (the date of
most recent publish was actually later) and all were unnecessary. I've been
removing copyright years piecemeal, for a long time, but I decided it was time
to just perform a bulk search and (mostly) replace. In some cases, I left
old copyright years on files that either are not used (and soon to be removed)
or obsolete and unlikely to ever be touched again (e.g. Win9x FOSSIL VXD). Some
of the runtime binaries still contain copyright years and those were updated to
2018.

_echo_off() functions.

Switched from #ifdef to #if defined.