Fix the problem that creating a user-defined type named _foo, followed by one

Fix the problem that creating a user-defined type named _foo, followed by one
named foo, would work but the other ordering would not.  If a user-specified
type or table name collides with an existing auto-generated array name, just
rename the array type out of the way by prepending more underscores.  This
should not create any backward-compatibility issues, since the cases in which
this will happen would have failed outright in prior releases.

Also fix an oversight in the arrays-of-composites patch: ALTER TABLE RENAME
renamed the table's rowtype but not its array type.