Unicode mapping scripts cleanup

Here is a series of patches to clean up the Unicode mapping script
business in src/backend/utils/mb/Unicode/.  It overlaps with the
perlcritic work that I recently wrote about, except that these pieces
are not strictly related to Perl, but wrong comments, missing makefile
pieces, and such.

I discovered that some of the source files that one is supposed to
download don't exist anymore or are labeled obsolete.  Also, running the
scripts produces slight differences in the output.  So apparently, the
CJK to Unicode mappings are still evolving and should be updated
occasionally.  Next steps would be to commit some or all of these
differences after additional verification, and then update the scripts
to use whatever the non-obsolete mapping sources are supposed to be.