maint: Add function insert_tautological_statement (ncl, conn, parent)

* dietncl/filter/prenorm4.lua: Add maintenance comment concerning max and
min.
* dietncl/filter/prenorm5.lua: New function insert_tautological_statement
(ncl, conn, parent); Use new function instead of 'update bind' in function
filter.apply (ncl).
* tests/test-filter-prenorm5.lua: Likewise.