Fix pg_dump to work against pre-9.0 servers again.

Fix pg_dump to work against pre-9.0 servers again.

getBlobs' queries for pre-9.0 servers were broken in two ways:
the 7.x/8.x query uses DISTINCT so it can't have unspecified-type
NULLs in the target list, and both that query and the 7.0 one
failed to provide the correct output column labels, so that the
subsequent code to extract data from the PGresult would fail.

Back-patch to 9.6 where the breakage was introduced (by commit 23f34fa4b).

Amit Langote and Tom Lane

Discussion: <0a3e7a0e-37bd-8427-29bd-958135862f0a@lab.ntt.co.jp>