Port changes from OpenAxiom commit 2826

The category AbelianMonoid has a new export: opposite?.
The category Rng has a new export: annihilate?.

git-svn-id: https://fricas.svn.sourceforge.net/svnroot/fricas/trunk@1543 b0c55286-4f34-0410-a049-a1e7e93b0762