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.

Modified Files:
--------------
    pgsql/contrib/dblink:
        dblink.c (r1.95 -> r1.96)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/contrib/dblink/dblink.c?r1=1.95&r2=1.96)
    pgsql/contrib/dblink/expected:
        dblink.out (r1.28 -> r1.29)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/contrib/dblink/expected/dblink.out?r1=1.28&r2=1.29)