Fill in the gaps in the GB18030 linear table.

The file gb-18030-2005.ucm used to generate this table was missing
certain ranges of the algorithmically-generated codes (otherwise,
the file would have been huge).  This adds all of the missing ranges
that map to within the BMP (including those that map to the PUA).
Clean up some code related to GB18030 (no table changes).

The table has been returned to rectangular format (see the previous
commit for functional changes).  Also, remove the #ifdef'd code for
mapping beyond U+FFFF, since all of those characters also exist in
the PUA and probably only appear in ancient Chinese documents.