pgsql: For 8.0 servers, get last built-in oid from pg_database

For 8.0 servers, get last built-in oid from pg_database

We didn't start ensuring that all built-in objects had OIDs less than
16384 until 8.1, so for 8.0 servers we still need to query the value out
of pg_database. We need this, in particular, to distinguish which casts
were built-in and which were user-defined.

For HEAD, we only worry about going back to 8.0, for the back-branches,
we also ensure that 7.0-7.4 work.

Discussion: https://www.postgresql.org/message-id/flat/20160504183952.GE10850%40tamriel.snowman.net

Branch
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REL9_5_STABLE

Details
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http://git.postgresql.org/pg/commitdiff/94476436a627d80319d49afe5724633b781e735a

Modified Files
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src/bin/pg_dump/pg_dump.c | 36 +++++++++++++++++++++++-------------
1 file changed, 23 insertions(+), 13 deletions(-)