pgsql: Fix user mapping object description

Fix user mapping object description

We were using "user mapping for user XYZ" as description for user mappings, but
that's ambiguous because users can have mappings on multiple foreign
servers; therefore change it to "for user XYZ on server UVW" instead.
Object identities for user mappings are also updated in the same way, in
branches 9.3 and above.

The incomplete description string was introduced together with the whole
SQL/MED infrastructure by commit cae565e503 of 8.4 era, so backpatch all
the way back.

Branch
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REL9_2_STABLE

Details
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http://git.postgresql.org/pg/commitdiff/e166e6441f14f612489706ca793cfd19f15d5951

Modified Files
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src/backend/catalog/dependency.c           |   10 ++++++++--
src/test/regress/expected/foreign_data.out |   24 ++++++++++++------------
2 files changed, 20 insertions(+), 14 deletions(-)