Improve psql \d output

Example:

test=# \d test
     Table "public.test"
 Column |  Type   | Modifiers
--------+---------+-----------
 a      | integer | not null
Indexes:
    "test_pkey" PRIMARY KEY btree (a)
Check Constraints:
    "$2" CHECK (a > 1)
Foreign Key Constraints:
    "$1" FOREIGN KEY (a) REFERENCES parent(b)
Rules:
    myrule AS ON INSERT TO test DO INSTEAD NOTHING
Triggers:
    "asdf asdf" AFTER INSERT OR DELETE ON test FOR EACH STATEMENT EXECUTE
PROCEDURE update_pg_pwd_and_pg_group(),
    mytrigger AFTER INSERT OR DELETE ON test FOR EACH ROW EXECUTE PROCEDURE
update_pg_pwd_and_pg_group()

I have minimised the double quoting of identifiers as much as I could
easily, and I will submit another patch when I have time to work on it that
will use a 'fmtId' function to determine it exactly.

I think it's a significant improvement in legibility...

Obviously the table example above is slightly degenerate in that not many
tables in production have heaps of (non-constraint) triggers and rules.

Chris