pgsql: Fix several problems in pg_dump's handling of SQL/MED objects,

Log Message:
-----------
Fix several problems in pg_dump's handling of SQL/MED objects, notably failure
to dump a PUBLIC user mapping correctly, as per bug #5560 from Shigeru Hanada.
Use the pg_user_mappings view rather than trying to access pg_user_mapping
directly, so that the code doesn't fail when run by a non-superuser.  And
clean up some minor carelessness such as unsafe usage of fmtId().

Back-patch to 8.4 where this code was added.

Modified Files:
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    pgsql/src/bin/pg_dump:
        pg_dump.c (r1.581 -> r1.582)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/bin/pg_dump/pg_dump.c?r1=1.581&r2=1.582)