replace 'std(..,hilb,..)' calls by 'groebner()' calls in 'primdec.lib'

replace std(..,hilb,..) calls by groebner() calls in 'primdec.lib'

reason:  additional a posteriori check (which?) is required, which is existant in groebner(), 
but is usually missing in 'primdec.lib' code.

Symptoms:
bugs like http://www.singular.uni-kl.de:8002/trac/ticket/662