Change the interpretation of the primary_key_attnums parameter of

dblink_build_sql_insert() and related functions.  Now the column numbers
are treated as logical not physical column numbers.  This will provide saner
behavior in the presence of dropped columns; furthermore, if we ever get
around to allowing rearrangement of logical column ordering, the original
definition would become nearly untenable from a usability standpoint.
Per recent discussion of dblink's handling of dropped columns.
Not back-patched for fear of breaking existing applications.