PATCH: Fix bug in inherited column definition

Hello!

Please see attached screenshot and a patch to fix that.

There are 6 system columns (tableoid,cmax,xmax,cmin,xmin,ctid) which
are counted before inherited columns go.
Steps to reproduce:
 1. Connect to database
 2. Create table parenttable (f1 integer, f2 integer,... and so on)
 3. Create table childtable inherits (parenttable)
 4. In "Options - UI Miscellaneous" enable "Show System Objects in the
 treeview".
 5. See definition of childtable.

Also free up memory taken by pgSet *inhtables.

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Best regards,
 J.F.