COMMENT ON mega patch

This patch does the following:

1. Comment on 5 new objects:

COMMENT ON CONVERSION my_conv IS 'Conversion to Unicode';
COMMENT ON LANGUAGE plpython IS 'Python support for stored procedures';
COMMENT ON OPERATOR CLASS int4ops USING btree IS '4 byte integer
operators for btrees';
COMMENT ON LARGE OBJECT 346344 IS 'Planning document';
COMMENT ON CAST (text AS int4) IS 'Allow casts from text to int4';

2. Dump conversions with pg_dump

3. Dump comments on conversions, languages, opclasses and casts with pg_dump

4. Upgrade psql's lob interface to use COMMENT ON instead of hacky insert.

5. Make large object comments get automatically removed when the object
is lo_unlink'd, so clients don't need any special code.

6. Documentation updates for (1)

7. Regression tests for commenting on all objects (currently COMMENT ON
is strangely absent from the regression tests, as this shows:
http://www.alcove.com.au/pgregress/commands/index.html )

Chris