typeTypeName() must return a pstrdup'd copy of the type name, not a

direct pointer into the syscache entry for the type.  In some cases
the syscache entry might get flushed before we are done using the
returned type name.  This bug accounts for difficult-to-repeat
failures seen when INSERTs into columns of certain data types are
run in parallel with VACUUMs of system tables.  There may be related
problems elsewhere --- we need to take a harder look at uses of
syscache data.