Fix up pg_dump's treatment of large object ownership and ACLs. We now emit

a separate archive entry for each BLOB, and use pg_dump's standard methods
for dealing with its ownership, ACL if any, and comment if any.  This means
that switches like --no-owner and --no-privileges do what they're supposed
to.  Preliminary testing says that performance is still reasonable even
with many blobs, though we'll have to see how that shakes out in the field.

KaiGai Kohei, revised by me