Fix GB18030 encoding support. And some cleanup ...

* Fix GB18030 Chinese encoding support for two and four bytes long
  characters. This finally resolves issue described in #6227;
* Processing of multi-byte characters was slightly refactored too;
* Remove the multi-byte 94/96 graphsets designation support for
  Japanese encodings. That looks like MuTerm rudiment, it had incomplete
  implementation and looked like abandoned. On the other hand multi-byte
  designation must be implemented in the same way as designation for
  single-byte graphsets was done. Note that this multi-byte graphsets
  designation has nothing to do with the normal encoding support for
  usual data flow conversion - so you will be on the safe side when
  use terminal encoding menu switch.
  The removed feature is the ancient technique to achieve different charsets
  support on 8-bit serial lines by assigning (designating) predefined
  sets of characters to G0, G1, G2 and G3 and selecting them during
  program life-time into GL (x20-x07E) or GR (xA0-xFF) areas by using LS
  or SS functions.
  For example xterm has no support for designation multi-byte graphsets
  at all. Anyway if this feature is required and you can provide the
  test environment - please let me know and I will be glad to implement
  this feature in more easy and consistent way;
* Remove unreferenced gSmbcsTable and gScsTable parsing tables that
  looks like is not used anymore;
* Remove gCS96GroundTable and gMbcsTable parsing tables that were used
  by multi-byte 94/96 Japanese graphsets support and now obsoleted by
  removing mentioned feature;
* Remove some obsoleted #defines, like HW statusline support for
  example, from parse tables definition.