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_0_STABLE

Details
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http://git.postgresql.org/pg/commitdiff/71b8e8e6cf12fb550977c4debbef036f90a94734

Modified Files
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src/backend/catalog/dependency.c           |   10 ++++++++--
src/test/regress/expected/foreign_data.out |   28 ++++++++++++++--------------
2 files changed, 22 insertions(+), 16 deletions(-)