More UTF-8 fun:

- bstrlen() moved to sbbs_t, accepts an option pmode argument so it can account
  for UTF-8 encoded strings correctly
- JS console.strlen() now accepts an optional pmode argument (e.g. P_UTF8)
- Renamed sbbs_t::utf8_to_cp437 to sbbs_t::print_utf8_as_cp437
- Create/use msghdr_hfield() to perform UTF-8->CP437 conversions as needed for
  printing/copying UTF-8 encoded message header fields.
- Defined XTRN_UTF8 misc setting flag. If a message editor does *not* have this
  flag, it is assumed to *not* support UTF-8. Will likely use this for UTF-8
  doors as some point too (none known to exist, yet).