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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master

Details
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http://git.postgresql.org/pg/commitdiff/cf34e373fcf42239a73f36e3054d9e9fbdc1e0de

Modified Files
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src/backend/catalog/objectaddress.c | 30 ++++++++++++++++++----------
src/test/regress/expected/foreign_data.out | 24 +++++++++++-----------
2 files changed, 32 insertions(+), 22 deletions(-)