pg_dump: Refactor code that constructs ALTER ... OWNER TO commands

Avoid having to list all the possible object types twice.  Instead, only 
_getObjectDescription() needs to know about specific object types.  It 
communicates back to _printTocEntry() whether an owner is to be set.

In passing, remove the logic to use ALTER TABLE to set the owner of 
views and sequences.  This is no longer necessary.  Furthermore, if 
pg_dump doesn't recognize the object type, this is now a fatal error, 
not a warning.