fixed some bugs for zero and patch for unit ideal decomposition

contains bugfix for http://www.singular.uni-kl.de:8002/trac/ticket/488

fixes the unit ideal decomposition (http://www.singular.uni-kl.de:8002/trac/ticket/550)
by introducing new interface 'primdecGTZE' , and so on which to return an empty list for unit ideal input.
This is done to provide the new behaviour in addition to the old one, giving the chance to fix all 'primdec.lib' usages
in independent additional patches. The price is (temporarily) one additional level of indirection.