Fix assorted infelicities in collation handling in psql's describe.c.

Fix assorted infelicities in collation handling in psql's describe.c.

In \d, be more careful to print collation only if it's not the default for
the column's data type.  Avoid assuming that the name "default" is magic.

Fix \d on a composite type so that it will print per-column collations.
It's no longer the case that a composite type cannot have modifiers.
(In consequence, the expected outputs for composite-type regression tests
change.)

Fix \dD so that it will print collation for a domain, again only if it's
not the same as the base type's collation.