new SQL command: CREATE CONVERSION/DROP CONVERSION added

I have committed changes enabling new CREATE CONVERSION/DROP
CONVERSION SQL commands. You can issue these SQL commands, but they
just add/remove tuples to/from new system catalog pg_conversion.
Still needs lots of work...

Initdb required.

For those who wishes to try these (currently) useless commands, here
is the brief description:

CREATE [DEFAULT] CONVERSION conversion_name FOR for_encoding_name      TO to_encoding_name FROM function_name;

DROP CONVERSION conversion_name;

description:

CREATE CONVERSION creates new encoding conversion.  conversion_name is
the name of the conversion possibly schema qualified name(yes,
conversion is schema aware). Duplicate conversion name is not allowed
in a schema. for_encoding_name is the encoding of the source
text. to_encoding_name is the encoding of the destination
text. function_name is the C function actually performs the
conversion(see below for the function signature).

If DEFAULT is specified, then the conversion is used for the automatic
frontend/backend encoding conversion.

conversion function signature is as follows:
pgconv(    INTEGER,    -- source encoding id    INTEGER,    -- destination encoding id    OPAQUE,        -- source
string(null terminated C string)    OPAQUE,        -- destination string (null terminated C string)    INTEGER
--source string length) returns INTEGER;    -- dummy. returns nothing, actually.
 
--
Tatsuo Ishii