pgsql: Consolidate and improve checking of key-column-attnum arguments

Log Message:
-----------
Consolidate and improve checking of key-column-attnum arguments for
dblink_build_sql_insert() and related functions.  In particular, be sure to
reject references to dropped and out-of-range column numbers.  The numbers
are still interpreted as physical column numbers, though, for backward
compatibility.

This patch replaces Joe's patch of 2010-02-03, which handled only some aspects
of the problem.

Tags:
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REL8_3_STABLE

Modified Files:
--------------
    pgsql/contrib/dblink:
        dblink.c (r1.69.2.6 -> r1.69.2.7)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/contrib/dblink/dblink.c?r1=1.69.2.6&r2=1.69.2.7)
    pgsql/contrib/dblink/expected:
        dblink.out (r1.21.2.1 -> r1.21.2.2)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/contrib/dblink/expected/dblink.out?r1=1.21.2.1&r2=1.21.2.2)