Generate EUC_CN mappings from gb-18030-2000.xml, because GB2312.TXT is no
longer available.

Get UHC from windows-949-2000.xml, it's more up-to-date.

Plus tons more small changes. With these changes, the perl scripts
faithfully produce the *.map files we have in the repository, from the
external source files.

In the passing, fix the Makefile to also download CP932.TXT and CP950.TXT.

Based on patches by Kyotaro Horiguchi, reviewed by Daniel Gustafsson.

Discussion: https://postgr.es/m/08e7892a-d55c-eefe-76e6-7910bc8dd1f3@iki.fi