CREATE CONVERSION mostly works now

I have committed many support files for CREATE CONVERSION. Default
conversion procs and conversions are added in initdb. Currently
supported conversions are:

UTF-8(UNICODE) <--> SQL_ASCII, ISO-8859-1 to 16, EUC_JP, EUC_KR,
EUC_CN, EUC_TW, SJIS, BIG5, GBK, GB18030, UHC,
JOHAB, TCVN

EUC_JP <--> SJIS
EUC_TW <--> BIG5
MULE_INTERNAL <--> EUC_JP, SJIS, EUC_TW, BIG5

Note that initial contents of pg_conversion system catalog are created
in the initdb process. So doing initdb required is ideal, it's
possible to add them to your databases by hand, however. To accomplish
this:

psql -f your_postgresql_install_path/share/conversion_create.sql your_database

So I did not bump up the version in cataversion.h.

TODO:
Add more conversion procs
Add [CASCADE|RESTRICT] to DROP CONVERSION
Add tuples to pg_depend
Add regression tests
Write docs
Add SQL99 CONVERT command?
--
Tatsuo Ishii