pgsql: Have pg_dump in binary-upgrade mode properly drop user-created

Have pg_dump in binary-upgrade mode properly drop user-created
extensions that might exist in the new empty cluster databases, like
plpgsql.

Backpatch to 9.2.

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/3e00d332615be32d64bbb1f604a783fade3146c0

Modified Files
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src/bin/pg_dump/pg_dump.c | 10 ++++++++++
1 files changed, 10 insertions(+), 0 deletions(-)