pgsql: Check to ensure the number of primary key fields supplied does

Log Message:
-----------
Check to ensure the number of primary key fields supplied does not
exceed the total number of non-dropped source table fields for
dblink_build_sql_*(). Addresses bug report from Rushabh Lathia.

Backpatch all the way to the 7.3 branch.

Tags:
----
REL8_1_STABLE

Modified Files:
--------------
    pgsql/contrib/dblink:
        dblink.c (r1.48.2.5 -> r1.48.2.6)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/contrib/dblink/dblink.c?r1=1.48.2.5&r2=1.48.2.6)
    pgsql/contrib/dblink/expected:
        dblink.out (r1.16.2.1 -> r1.16.2.2)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/contrib/dblink/expected/dblink.out?r1=1.16.2.1&r2=1.16.2.2)
    pgsql/contrib/dblink/sql:
        dblink.sql (r1.15 -> r1.15.2.1)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/contrib/dblink/sql/dblink.sql?r1=1.15&r2=1.15.2.1)